User blog comment:Bovell/Lots of new Battlefield 3 details via Twitter, DreamHack festival, PSM3, and interview/@comment-71.239.231.154-20110704032843/@comment-1439572-20110705174839

I'm pretty sure enough bullets will stop the knifer from killing if he doesn't hit the target.

The health goes down to 0% only when he puts the knife in (Multiplayer teaser).