When playing K&L online (known as Fragile Alliance), you'll be part of a bank heist. Basically, your squad will try to grab as much loot as possible, taking out cops and other security measures all the while. If you notice that a teammate has a sack a bit fuller than yours (or you just want to pad your sack a bit more) you can, as they say on the streets, kill him. But once you do so, the whole team knows, and they likely won't take kindly to it. Even if you do get popped, you're still in the game, reincarnated as a police officer, allowing you to take delicious vengeance on your killer.
We don't know all the ins-and-outs of Fragile Alliance yet, but, suffice it to say that we can't wait to get our hands on it.