Actually, he was killing villains, not heroes. He was on the heroes team, therefore technically he was not killing his teammates.

Having said that, however, he did not engage villains in combat, but rather teleport them to a kill zone. That's unethical, for sure. Had he simply engaged villains, I'd see nothing wrong with that. If other players don't want to kill villains, even if that's their job as heroes, that's their problem.