in MGS4 you can sneak up behind ppl and do a variety of different things: hold them at gunpoint and have them empty themselves of ammo, etc.; choke them out; grab them around the neck and drag them/use them as human shield while u shoot at other enemies; or u can choose to just take ur knife and slice their throat. I think there are other options, but that's all i remember right now. U GOTTA check out MGS4, best game ive ever played alongside Uncharted 2.
Someone mentioned Uncharted 2...i think everyone and their mother MUST at least try Uncharted 2 just bc its an incredible game. As far as the stealth kills though, ur not violently stabbing ppl in the back; the stealth kills are all done via melee..u can take someone out around a corner in a variety of different ways; u can sneak up behind someone and jump on their back and break their neck; u can take someone out behind cover-grab them by their shirt, drag them ontop of ur cover, elbow them in the throat/chest, then drag them unconscious behind ur cover with u; and lastly u can pull someone off a ledge while ur hanging from it. This games great, and the stealth kills are an added bonus to an already amazing game. Oh yeah, and the multiplayer is ridiculous amounts of fun, the first game to get me into online multiplayer...
Oh yeah, and one last thing...someone also said Uncharted 1 had stealth kills. While I think u should still check out U1 bc its a good game, i don't want u to be misinformed. U2 is the first game in the series to have stealth kills.