If you try to justify the use of deadly force based on the possibility someone is about to slap someone else, that's bull**** and you will be called on it by just about anyone. Indeed if you hit someone before the first blow lands in any other circumstances, that's probably still going to be counted as assault though it might not be prosecuted.
Except in this particular case. Justifiable homicide, as in this law, deals with prevention, not with ex-post-facto reaction to an attack. That is the fundamental problem here, in that it lowers the bar on which you may kill someone to suspicion of intent to assault.
When I read that bill, I see it modifying the original clause to include fetuses, but not actually saying 'it is legal to kill someone on suspicion of intent to abort.' That seems to me to still keep the bill subject to the constraints higher courts have established, and prevents this law from meaning what everyone thinks it does.
I don't have nearly as much experience in these matters, but I'll try to add my two bits based on the law class I took last semester. The use of lethal force in the prevention of a felony is one of the messiest parts of US law. As I recall, in order for it to actually be used as a defense, there must be imminent danger that a reasonable person would be able to perceive. And the question of what qualifies as 'imminent' is messy, too. Usually it is interpreted to mean at the moment violence is initiated. It means if you're caught in an abusive relationship and your partner has explicitly said 'I'm gonna kill you tomorrow,' and you've seen the loaded gun they intend to use, you still have to wait until the gun is drawn in order for it to not be premeditated murder. You can't up, take the gun and shoot him or her in the middle of the night.
That said, it seems to me any vigilante trying to defend fetuses would get shredded in court, because they would be incapable of intervening right before the abortion occurs in a way that isn't blatantly premeditated. I feel their justification of use of deadly force would fail, because they'd pretty much have to camp outside the office with a high power rifle or other device capable of harming the building's occupants to intervene, and the amount of preparation that takes makes it perfectly clear that the use of force is not 'in the moment.'
Also, they'd need to have some *****in' x-ray scopes and **** because most clinics don't have windows in their treatment rooms.
Personally, this feels like legislative trolling, both against pro-choice and pro-life elements.
And holy christ you guys, 16 new posts while I was typing.

A heated topic indeed.