I'd phrase that as more "shoot to PREVENT someone from stealing my truck." I kind of am on your theoretical position there, in that while I might feel a spiritual obligation to try and peacefully prevent a thief from getting hurt and carrying out his crime, at the point where he becomes a danger to me or threatens me, or anyone, in order to carry out his crime, he's fair game. Obviously, you don't have the moral right to sneak up on a burglar in your house and put a couple rounds in his back, but legally, your property rights should preclude any legal judgement of what happens to someone who has illegally imposed himself on your property or in your domicile. Imprisonment, of course, being an exception.

Essentially, it is about the three natural rights of life, liberty and property. Your right to your own trumps that of anyone else's. In order to preserve your life, you have the right to take another's (who threatens or endangers yours), and you have the right to steal others' property (such as food or medicine, though with an accompanying duty to make restitution as soon as possible). You have the right to kill to preserve your (justified) liberty, and to defend your (rightful) property. Obviously, the social compact requires some inherent acknowledgement of the right of society to limit those for the greater good, such as being arrested for a crime or forfeiting property in restitution for the same. It is by this same logic that the confiscation of property, imprisonment and execution are justified punishments for crimes, as they are theoretically aimed at the protection of the life, liberty and property of others.

With that philosophy in mind, abortion is almost never justified. By the estimates of a pro-abortion group, there are something like 800 medically necessary abortions performed every year (and that stat was not cited in an "only 800" context, it was cited in a "how dare you deprive these 800 women" manner). Out of thousands. I can except abortion as an unavoidable consequence of a necessary medical procedure, but not as an end in and of itself. Things like "she's depressed and will probably kill herself if not permitted an abortion" only mandate a 120th trimester abortion to be performed on the "medical professional" who might possibly make such a statement. IMO, the death of a mother who would kill her baby to survive is not much loss to society or her theoretically beloved and loving family.
Regarding the whole rape issue, the circumstances of conception are immaterial. However unfortunate the mother's victimization, compounding it with the murder of a child, even if unwanted and unloved, is not justifiable. What I find inconsistent is the attitude of the nonreligious who don't believe in an afterlife being willing to sign off on the curtailing of a human being's only life. My own belief that the pain of the induced abortion is the last suffering that soul will experience for the rest of its eternal existence would seem to be more justifying of abortion than the willingness to consign that creature to oblivion before it ever gets a chance to experience anything.

Hope that helps.