We need to understand that our society frowns on one human intentionally killing or hurting another. However, our laws, going back to the Common Law of England on which our system is based (and even before then in other systems), recognize that there are situations in which an intentional act of extreme violence against another person can be justified -- for example when absolutely necessary to defend an innocent (whether oneself or another) against an otherwise unavoidable threat of being killed or gravely injured by another person's intention act.
The original rule was that before using force in self defense, one had a duty to retreat if he could do so in complete safety. And this did not apply in your home, because your home was your place of refuges; and no one should be able to force you to leave your place of refuges. And of course, the duty to retreat reflected the core societal value that intentionally hurting another human was inherently repugnant, and resort to violence was to be avoided when possible.
The real idea behind SYG laws is to avoid having to deal with a dispute about whether one could have safely retreated. That could often be a tough question. A difficult side question would be whether the actor, in the heat of the moment during a rapidly unfolding and dangerous emergent situation could even have been reasonably expected to have been aware of an available means of escape. So to have the protection of a SYG law, all other requirements need to legally justify your act of violence against another human still need to be satisfied.
And there remains the practical side of things. A fight avoided is a fight won.
Originally Posted by animal
...Life, liberty, and property are inalienable rights that are supposedly protected and guaranteed in the US Constitution....
An appeal to the Constitution is a red herring in this context. The Constitution regulates government and not private conduct. The private person attacking me is not violating my constitutional rights because he is not bound or regulated by the Constitution.