Is it ever okay to kill someone?

I think that, if you need to stop a person who is imminently about to kill or badly assault someone else, and in stopping that person, you kill him whether deliberately or by accident... I think that's perfectly fine.  This is 100% consistent with the non-aggression principle.

Note how  we do not need to resort to crazy bullshit like laws declaring killing "unlawful" or other religious scriptures declaring killing "a mortal sin", to deduce this obvious moral fact that killing in self-defense or defense of others is perfectly ethical.  In fact, as a matter of course, all religious texts nearly always include exceptions to murder, expressly designed to excuse the rulers anyway, so relying on them is simply error.

I don't, however, think that after-the-fact killing of a person (whatever the circumstances) is either ethical or accomplishes anything constructive. Not for the victim. Not for society. Not for the killer. I find it highly offensive to decent people's ethics, that a murderer is taken to trial, at the cost of four million dollars, then killed, and on top of this egregious charade, the victim and society are made to foot the fucking bill for this circus of false "justice". It's utterly retarded, insult on top of injury, designed to only appease the basest human beings.

We live in this world where the punitive, retaliatory, vindictive nature of our "justice" system is taken as a given that is entirely unquestionable. Once people start questioning it, and looking at retributive, defensive and restorative justice (the only true kinds of justice), the world will change; in other words, true justice will only come about once the ethics of non-aggression -- which almost all of us already live on a daily basis -- become crystal-clear to everyone.