Killing Newt and Hicks was a horrible idea, and not just because of how it hurts the ending of Aliens.I don't think it was necessary to kill Hicks and Newt. But he didn't do it out of spite or anything. He did it because he wanted to explore such a painful possibility.
Alien 3 explores the painful possibility of two characters dying who don't actually have any characterization in the movie. All the writing for Newt and Hicks that actually made the audience attached to them and explained their relationship with Ripley was in the previous movie. So either you haven't seen Aliens and you have no idea why you should care about these two characters, or you've seen Aliens and you're wondering why the movie killed them off in such a contrived and lazy way.
Killing characters without even developing them and expecting the audience to feel something isn't good writing, it's cheap shock value at its worst.