So I was in Megaton and wanted to get the achievement for putting a grenade in someones pocket. Without thinking of saving prior to doing this I did it and the whole town turns on me. What happens if I kill off  everyone after me? or should I just not go back?

If you guard and try to talk someone, they might accept your apology, but I don't know if that'll work with something like that...

ya its kind of hard to do that when being shot at by groups of people

Return after 72 game hours and they will no longer attack you.

Or just activate the nuke and blow them all to hell. They did attack YOU after all... I mean, the grenade thing was just an accident :-X

Leave the town and return after two or three in-game days and the town will no longer be hostile. If you're just out of the Vault I suggest heading South and trying to locate Rivet City, although you could just sleep for two days outside of the Megaton walls (but that's no fun).

Lies look at how old this topic is i really doubt GoodWood is even interested anymore ....

"Lies look at how old this topic is i really doubt GoodWood is even interested anymore ...."

Yeah, if possible, it'd be a good time to nuke 'em. Or I guess I should say a month ago would have been a good time to nuke 'em.