Because you lose about 4x as much Karma as you'd get back from killing Hightower, his wife, his guards, and turning in the quest. I understand it's a power gaming move. Fallout 1 already feels small, so few quests. Just one less to do if you're trying to get XP & caps, while maintaining high karma. Side note. When attempting to disarm Hightower's chest, I triggered the bomb and the game crashed. Also, in The Glow. I was sometimes able to bypass the security card check on the doors by simply clicking to walk inside them, since doors open automatically. Wasn't consistent, not sure why it allowed me only sometimes.