Sure it could, and is. Again, most states allow the use of deadly force against intruders. But there are enough cases out there of some innocent fool getting his head blown off by trying to enter the wrong house in the middle of the night drunk, etc. Shooting someone in such circumstances is murder by my understanding of the word, and the corner will classify the death as a homocide.
You are confusing morality and law. I agree that abortion should be illegal. In fact, it is illegal in most states, other than the fact that prosecutions for the crime will be thrown out under Roe vs. Wade. But we do ourselves no credit by exercising wishful thinking to create things int he law that are not there.
I agree with you also that abortion is murder and we need to work to change the positive laws. Indeed, even if the fact that abortion is murder is not in a man-made law, it is still a law of nature and natures God. Although I dont agree that cases of self-defense are ever categorized as murder. There is a moral "double effect" there. When you resort to self defense and the attacker gets killed, your intent was not to kill him, it was to defend yourself. Morally and legally, that is not murder.