I guess one question for me is "necessary for what, and in what situation?"

If it's necessary for me to kill someone in order to rob a liquor store, or escape after a robbery, no, it's not justified.

If it's necessary for me to kill someone who is threatening me to stay alive, sure, it's justified.

And of course there are grey areas. How serious is the threat, etc. Wondering whether I'd misjudged the seriousness of the threat, or whether I could have come up with a less extreme way of solving the problem, would probably keep me up at night.