What's the first game in which you can dismember enemies?
Chiller (arcade, 1986) would be an early example,:
http://en.wikipedia.org/wiki/Chiller_(video_game)
But the people in that game are only "enemies" in the sense that your goal is to shoot them (they cannot harm you or fight back in any way), and I'm not sure if it's the first anyway.
Edit: now that I think about it, Asteroids (arcade, 1979) might be the progenitor to all enemy dismemberment, since you blow apart the rocks into multiple pieces with each shot. Of course, they are still just rocks.