"to harm the unborn child of such person in a manner and to a degree likely to result in the death of the unborn child, "
Note, doesn't say "harm the unborn child" illegally...but simply says harm it. So a doctor performing a legal 1st trimester abortion...hell, a person in a clinic giving the person the morning after pill...could be concievably killed under "justifiable" homicide for doing a completely lawful act that is part of their job.
How in the world is allowing people performing a legal action that is part of their job be legally murdered a "good" thing. Even if you think abortion is a bad thing, legalizing further murder isn't a solution.