Well, I have to go to many years ago, when Left 4 dead 2 broke all the records for its amazing coop mode. I played it on XBOX 360 (yeah, I am that old); if you survive the whole chapter, at the end a vehicle comes to "rescue" the survivors (like in helldivers 2); I was playing with three random dudes, when we were about to escape, one of them decided to go out of the vehicle to kill more zombies. His life was very low and then I saw THE chance: I went out of the vehicle and BAM!! Headshot with my shotgun...without him noticing until it was very late. When you lose all your life, your character was on the floor waiting to be healed by other player...but we decided to go back to the vehicle and left him there to be killed by the zombies.
We all were laughing hard while they guy was shouting and insulting me in ways that I have never listened.
Welcome to the cruel world of Left 4 dead 2, baby.
(I used a video of another game with friendly fire on).