So yeah playing through the single player again and I still dont fully understand why Sheppard killed Roach and Ghost, he wanted to cover up everything he had done?

Basically, although someone else will give you a more in-depth reason.

" Basically, although someone else will give you a more in-depth reason. "

I shall try.
Remember that the team split up.  Roach and ghost go to Russia to get the data on Makarov back which will show that Makarov was behind the massacre at the airport.
Well i have a couple of reasons behind his killing of Roach and Ghost and planned murder of Price and Soap, since from the radio communications, they found out he was a traitor.
First off, he had wanted this war between Russia and America to start, by having Russia be the invading force.  To do this he puts Allen in with Makarov and I believe that he compromised Allen's identity so as to allow Makarov to set it up to look like American's were responsible.  And I think that this fact had evidence in the data retrieved from the house.  Part of me believes that Makarov had been working for Shepard to set this war up, because they make it a point to let the audience know he is a mercenary and no revolutionary.  Also, how does Makarov know the location of Shepard's base in Afghanistan?

Also, because he was so pissed about what happened in COD4, he wants to make sure America gets credit for getting the evidence on Makarov, not the British agents of the 141.  So he wastes them to get rid of the evidence and make sure only he gets credit for blaming Makarov.

" @TheHBK: A question I have: Why was Allen chosen to be the undercover guy at the airport? He seemed like just any soldier. "

My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game.

Yep, he's just eliminating anything that can possibly lead back to him starting the war.  He also doesn't want anybody to find out how the nuke got launched, because that was key to getting his blank cheque.
 
Shepherd employing (or at least working with) Makarov is the only explanation that really makes any sense.  Makarov's plan relies entirely on being able to leave an American agent at the scene of the crime; where else is he gonna get one if not from Shepherd?  No agent, no point in the attack on the airport in the first place, because it wouldn't achieve anything.

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

" @CornontheCobbe said:

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

Price fired the nuke and had it detonate prematurely in space.  The EMP blast from the nuke disables all of the Russian hardware, allowing the US troops to easily win the battle and repel the invasion.  The nuke is far enough away not to do any real damage to the city. "

No there wouldn't be as it was detonated in space so that radiation wouldn't reach the surface, only the emp effect would reach the surface of earth.

I want to know why they only poured gasoline on Roach i guess his name was, and not on that tard Ghost? (i might be wrong but i don't remember them pouring it on Ghost or setting him on fire)

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

Geez, I thought people would have figured everything out already. The story isn't that hard to understand. Listen to the Modern Warfare 2 Bombcast.

" @CornontheCobbe said:

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

yeah i never got why he let the sub fire em at the end soap said THE PORTS ARE OPENING and price said good "

"

Geez, I thought people would have figured everything out already. The story isn't that hard to understand. Listen to the Modern Warfare 2 Bombcast.

"

As was already said, Price wanted to EMP the Eastern Seaboard (Well, D.C./Virginia, where Russia was focusing it's invasion), thereby killing power to everything. Since it was home soil, the US troops had the advantage since there was no electricity running (apart from the White House). With the playing field "leveled", so to speak, the US troops could win.
 
The whole ISS/Astronaut blowing up in space/getting thrown is physically impossible, but still makes for a badass scene. It's the Radiation of the nuke that effects particles in the atmosphere, which creates the EMP, but it's so far up that deadly radiation levels don't appear on the surface.

" @nanikore said:

" @TheHBK: A question I have: Why was Allen chosen to be the undercover guy at the airport? He seemed like just any soldier. "

Cause unlike most of the guys you played as in the COD series, he actually talked. :D  Nah, probably because game developers thought that players were going to get good times on the course and so Shepard would see the Allen was a good shot, backed up by the fact that Dunn said something along the lines of this in the beginning. "

" @Metroid545 said:

" @CornontheCobbe said:

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

yeah i never got why he let the sub fire em at the end soap said THE PORTS ARE OPENING and price said good "

Because he was firing the nuke to set off an EMP blast over Washington D.C. "

" I shall try. Remember that the team split up.  Roach and ghost go to Russia to get the data on Makarov back which will show that Makarov was behind the massacre at the airport. Well i have a couple of reasons behind his killing of Roach and Ghost and planned murder of Price and Soap, since from the radio communications, they found out he was a traitor. First off, he had wanted this war between Russia and America to start, by having Russia be the invading force.  To do this he puts Allen in with Makarov and I believe that he compromised Allen's identity so as to allow Makarov to set it up to look like American's were responsible.  And I think that this fact had evidence in the data retrieved from the house.  Part of me believes that Makarov had been working for Shepard to set this war up, because they make it a point to let the audience know he is a mercenary and no revolutionary.  Also, how does Makarov know the location of Shepard's base in Afghanistan?Also, because he was so pissed about what happened in COD4, he wants to make sure America gets credit for getting the evidence on Makarov, not the British agents of the 141.  So he wastes them to get rid of the evidence and make sure only he gets credit for blaming Makarov. "

  hey guys just thought i'd shout in and say yeah what price does it totally possible (and has happened before)
The physics behind it is, the bomb detonates in the atmosphere and as a result produces a lot of gamma rays. The photons from the gamma rays produce high energy, delocalised electrons via a process called Compton scattering. These electrons are trapped in the earth's magnetic field, which then leads to an oscillating electric current, which due to its asymmetry leads to a radiated electromagnetic field which is the emp

how price knew this? spoiler alert he's Gordon Freeman

I think he either did it for the dope or revenge.

The story in MW2 isn't explained all that well so there are some inconsistent holes in the plot. Basically the reoccurring theme in MW2 is how history remembers the heroes, how history remembers us and how we are written and perceived in the history books.
 
Shepherd, in short, is pissed that during the war in COD4 he lost thousands of troops during the nuclear blast in an effort to stop Zakhaev from winning, yet even after all those sacrifices and struggles. Zakhaev and the Ultranationalists still win, Zakhaev is branded a martyr and hero of Russia. The Ultranationalists rise to power in Russia and the world just stands idly by and lets it happen without batting an eyelid.
 
So when he sees the chance to claim responsibility for finding the incriminating evidence proving that Makarov, a terrorist and former member of the Ultranationalist Russian party, is the man responsible for the "No Russian" fiasco and subsequent Russian invasion of America he grabs it. He betrays Task Force 141 so that he, Shepherd, can claim the glory and this time have history written the way it should have been, with the terrorists (Ultranationalist Russians) being branded the villains and not the heroes. And that those who sacrificed themselves to try and stop people like Zakhaev, Makarov, Al Asad and the terrorist Ultranationalists will not have sacrificed themselves in vain, and that they will get the recognition of victory in the history books like they deserve.
 
In short Shepherd basically realises he has a chance to right, what he considers personally, to be many wrongs and all it takes is silencing the very few people that know the truth, a price he is willing to pay for the greater good and grander scheme of things. The moment he gets that 'blank cheque' Shepherd realises he has everything he needs to wage a fully justified and legitimate war with unlimited resources that will put his name in the history books forever.

" @Metroid545 said:

" @CornontheCobbe said:

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

yeah i never got why he let the sub fire em at the end soap said THE PORTS ARE OPENING and price said good "

Because he was firing the nuke to set off an EMP blast over Washington D.C. "

" @Metroid545 said:

" @CornontheCobbe said:

" My question was why did Price not stop the nuke from going off into space? I didnt really get that. I thought he was on Sheppards side when he let it fire... I'm kind of confused at what happened in that game. "

yeah i never got why he let the sub fire em at the end soap said THE PORTS ARE OPENING and price said good "

Because he was firing the nuke to set off an EMP blast over Washington D.C. "

Its not that hard to comprehend. A high altitude detonation of a nuclear weapon creates the mother of EMPs. It would have devastated electronics all across the coast, giving the US military the upper hand because its on their turf.  
 
I personally love the fact that they used that in the game, because its not a common use of nuclear weapons in fiction.

DO YOU REALLY WANT TO KNOW WHY?
 

 Listen to theBombcast on Modern Warfare 2. They throw good theories around. The one that I like is that Shepard saw the opportunity to bring meaning back to the army after the blow it took from the nuke detonating in MW 1, so he wanted the war to continue and by killing everyone involved in the Task Force 141, no one will find out that the U.S was innocent. Reasons to support this is that the quotes towards the end of the game that appear when you die always mention something about extreme patriotism.

What I don't get is why didn't the Russians just nuke the Americans if they wanted rid of them and why didn't the Americans just nuke the Russians when they invaded mainland America?
 
In other words what happened to mutually assured destruction?  

" What I don't get is why didn't the Russians just nuke the Americans if they wanted rid of them and why didn't the Americans just nuke the Russians when they invaded mainland America?
 
In other words what happened to mutually assured destruction?  
"