Because killing a person deprives them of positive experiences that they otherwise would have had, and they prefer to have them. But a nonexistent being doesn't have preferences.
Once you've killed them and they've become nonexistent, then they don't have preferences either.
Haven't had one of these for awhile. This thread is for questions or comments that you've felt silly about not knowing/understanding. Let's try to exchange info that seems obvious, knowing that due to the illusion of transparency it really isn't so obvious!