That's just plain asinine. Harming the property of one who initiated aggression, while in the act of self-defense is clearly not only "okay," but a moral imperative, in the event that it justly saves a life.
I want to know why.The burden of proof is on the one who takes action, you. You wish to damage their property and I wish to know how this is reconciled with the principle of private property. You do not own their property or body, but taking direct physical action against them is you taking control over their property against their wishes. I see that as absolutely inconsistent with the most basic libertarian values.
The reason is because the basis for morality is
life itself. If you're going to set up a system of rules to protect the thing that matters, you're not going to disarm people from protecting it. Besides, in your silly example, I'm
not the one who takes the action. The person initiating force is. I'm responding. You claimed I "wish" to damage their property. I never cited a "wish" to do so. When a person aggresses, he forfeits his rights to the extent necessary to defend against the aggression, because this is what is necessary to defend life and property in the first place.
You need to study ethics. While I don't see Ayn Rand as right about everything, she hits the nail on the head in the first chapter of her book: "The Virtue of Selfishness," "The Objectivist Ethics," in which she sets up the only rational framework for ethics. You could also learn a lot from the chapter "The Ethics of Emergencies," in the same book.