Now, before you start screaming, I am NOT at all trying to claim that genocide in a game is bad. I do it all the time. I like it. We should keep it. Especially since if you kill a virtual being, he'll just respawn. So nothing bad is really happening. The End.
The main question is, Why? Why do we like killing virtual people? Why is it fun?
For me, it's the idea of having power and strength...and actually getting what you want. Er, I know that doesn't sound that great, but we are virtually powerless in real life, so that's about it.