In my Green Zombie post, I mentioned several possible reasons for the popularity of zombies in games. But I've heard the most sensible reason yet in episode 90 of the RetroforceGO! podcast. At 31:30, Reverend Anthony says this about shooting a zombie:
"I might be, like, screwing myself over by saying this, but it gets you the benefits of, like, shooting a person without any guilt."
Sometimes the simplest explanation is the best one.