As a non-american, it boggles me that people use lethal force so lightly. Not even in situations where you are in danger yourself it is automatically warranted to use lethal force, only the force that is appropriate to deflect the danger is allowed, in general. Now of course there is some wiggle room here, and of course there is also the heat of the moment, but for example when someone is hitting you with a fist and you just blow his brains out, you are committing a crime too.
I don’t think it matters a lot if she was actually pregnant or not. He was ready to shoot her anyway, he didn’t care.