Well many games at have teamkilling enabled so at least that shows you need to be careful where you shoot.
Some advanced hardware (aircraft, tanks) keeps you from shooting friendlies.
I've even seen advanced rifle prototypes that do the same.
There are plenty of players who want to kill everyone in video games.
I've heard there is at least one game that gives you a "badge" if you manage to kill every living thing in the game.
I've also heard there's a game where if you kill too many people, the game economy collapses. lol