We've got a couple of clips that are somehow related to Valve games, and a bonus MW2 clip for those who haven't seen it yet. Go!
For future reference: the AI in L4D can be quite scary.
...Well, if you consider killing zombies to Counter-Strike, you must die pretty easily.
On that note, here's yet another person pissed off at CS.
BONUS: Awesome Modern Warfare 2 knife kill.