It's a game, and it's not huma... err, gamer nature to be helpful when there's easy killing to be done.
Some folks (more than ever before it would seem) are not the least bit interested in you, your team, staying alive, being tactical, being smart... whatever. They get thier jollies watching things go boom, all else is trivial.
I call it the Quake Effect.
It is bad.
but
Build a game..... and they will come (infest is a better term really)