Pretty much self explanatory.   
 
 Mine would definitely be a quote from my all time favorite books, The Book of Five Rings.  
      "Perceive that which cannot be seen with the eye."     Miyamoto Mushashi    

Goodnight sweet prince, obviously.

I heard you like Mudkips.

just to get it out of the way:
"UH UH UH UH UH"
 
sorry

"Assassination, ho!" 
 
~! Edited to fit the scenario.

"I'm sorry we couldn't work out our differences in a manner that was more agreeable with your spine."

I'm sorry

oops

"your peach fuzz makes me happy" 
 
or 
 
"just imagine its my organ"

"You know, you poop yourself when you die."

'I PITY DA FOO. Requiescat in pace.'

"like tear drops in the rain"- Or something like that^^

"Nice ass"

You gonna kneel, but your not gonna pray!  Then I would look down at my crotch and say "Stand and deliver"

Probably something along the lines of: 
 
"You have that look in your eyes of a man who is bewildered, as if the sudden realization that a universe you thought belonged to you suddenly crashed around, revealing in it's empty abyss that you're not the center upon which it revolves.  Take a moment then to reflect what you used to think were achievements in life, all the vain goals you've striven for, only to have them all taken away by a person you don't know with motives you're to afraid to ask.  Perhaps once you finally accept that these goals were as equally vain before your impending demise as they are now, you'll find some piece on the other side to solace your broken so--, wait, are you still alive?  Well fuck me, I've been sitting here for some thirty odd minutes talking philosophy with a corpse.  Now there is irony."

"I am the walrus. Goo Goo gachoo. "

"I took you like my woman... from behind."

I would recite this entire song. 
 
  

"WHAT!!!!!!"

Suck it or #&ck it.

"Wildcard .....Bitches! *cowboy laugh* "

"Dosvedanya," with a Russian-Martian accent.

make a really obnoxious slurping noise

 “I love you but I am not IN LOVE with you."  

There needs to be a feature in this game where you can record like a 5-10 second audio clip of the player saying their assassination line of choice and it will play everytime you are killed by the player in a stealthy manner.

 "Yippee kai yay, motherfucker!"     
 
  

"Eat several dicks!"

"Your corpse will satisfy my sexual fetish :)"

Die. Would you kindly?

"Y'all be gettin assassinated up in here"

'After this, I'm going to bang your mum'
 
Pretty crude, but would completely mentally crush anyone who I was murdering

"Tune into Conan.  Weeknights on TBS."

" I heard you like Mudkips. "

 Vieni qui cagna e succhiare il mio pene    

look. a three headed monkey

"Remember when I said [I loved you, you could trust me, I'd kill you last, etc]? I lied."

"When I'm done killing you, I will sexually defile your corpse and then hunt down your wife and children, horrifically raping and murdering them before tearing their bodies asunder and casting the bones unto the cruel sea; so their souls may never find respite in the Hell that I sent them to."
 
or
 
"gg"

"Thank you"

you dead, motherfucker

" I would recite this entire song. 
 
  

    Or if I'm feeling lazy I'll say ''Game Over'' "

"All your base belong to us"

Right behind you *le backstab*
 
Nah, but maybe "DIE ONE THOUSAND DEATHS"

slayer!

"Game over man, game over!"

(This is not my knife. I'm actually happy to see you.)

My secret name is PoopDickPappy...

"I just saved a bunch of money on my car insurance."