[Mattk]

There are many exceptions to trespass. For instance, you can go onto other people's yards to pick up runaway balls, dogs etc. I doubt any local politicians get shot during their door-knocking rounds. Simply walking onto someone's front yard is not threatening unless they are carrying implements or weapons. Your belief of threat has to be reasonable. It is an objective-subjective test. If you saw a bored police officer approaching your door, ordinary people would not immediately apprehend a bad dose of police brutality and start reaching for their guns, and if you did, you would be being unreasonable. I will concede that in the case of David, this was not the case, but uniformed police officers tend not to engage in indiscriminate slaughter of the general populace.

In the case of a break in, you can definitely get away with killing people. However, why kill if you don't have to? Most burglars aren't killers. They just want to steal electronic goods to hock so they can buy drugs. If the going gets tough, they go somewhere else. If the police start taking all your stuff illegally, it is not going to kill you. You'll get the stuff back after they realise they fucked up and can't use it. If the police want to arrest you, just cop it or they will definitely engage in some police brutality. There is no point causing controversy, because even if you are justified in killing someone, it is up to you to prove it.

I fully understand the principle of self-preservation. It is a life-saving principle, where you put yourself out of harm first before attempting to rescue others. It has nothing to do with pre-emptive killing.