I think for pro-abortion it is more about letting the woman decide to undergo an intervention over something which will affect her health/well-being significantly. So killing a fetus/baby might still be a certain amount of bad (maybe ramping up continuously with age), but it is more bad to not allow this choice (but this is de-emphasized by the pro-abortion movement for the obvious political reasons). I think this also explains why lots of people are ok with early-term abortions, but not late-term abortions.