Look up castle doctrine.

It's the laws that we have. Basically, if someone breaches your home, you're under no obligation to back down and if you respond with lethal force you have a lot of the benefit of the doubt, fear is implied in that situation. The police are treating it as if they were breaking in to one of their own homes, because if you or I did that, we'd get shot and killed. The only difference it that they have a legal document that allows it in that case, you have to serve that document though.

I had a coworker that lived on a street like "N 13th Avenue" and I guess there was some sort of crackhouse at "S 13th Avenue" one night the police served a no-know warrant, he was pissed and demanded to know what was going on, they shot his dog and killed his dog, he was shot in the hip and had permanent damage from it, his wife was marched out in cuffs half naked in-front of the neighborhood. When the dust settled the police realized they went to the wrong address. The police reached some sort of settlement with them, it never seemed remotely fair. (Think $700k in the mid-1990s) He was in his 50s when I worked with him, this happened in his late 30s or early 40s. He looked like he was 80 though, walked with a cane. He ultimately passed away at a fairly young age.