Think about the kind of life you think would have been better off not existing. Surely there are people alive whose life fits your criteria. Now imagine telling one of these people that it would have been better if they never existed.

Probably they'll tell you where you can stick your notions.

I'm not suggesting someone goes and tells them, that would just cause more pain. I'm suggesting we stop that life from existing in the first place.

If you have other reasons for not wanting a life to exist, that’s another matter. But if your only reason is that, in your estimation, that person would have a life not worth living, that seems awfully presumptuous. Life is optional. Let them decide if they want to be alive.