Killing or murder?
Killing to defend innocents seems pretty justified. If some crazed person with a gun is attacking a school, it seems that there should be no concern about killing the crazed person with a gun.
Self defense is also a valid reason to kill someone else.
The problem becomes whether there was a credible, realistic threat or not. If someone merely imagines that their life is threatened by someone else, they could engage in what they term "self-defense" and not be engaged in self-defense at all. Many of the antifa and no-platform types justify their behavior on the basis of self-defense because the mere utterance of words contrary to their narrow worldview is an attack on them personally.....
Now, governments use both excuses to attack other countries and allege they are doing it protect innocents or in self-defense, which is largely the just war doctrine. "The unstable country is causing all kinds of problems for the whole world so let's go kill a bunch of the people there and install our own cronies!" or "They attacked us (but ignore all the things we did to attack them before that)" type deals.
I view most of those as bogus and manufactured, and most have a long list of provocations.
So, tl;dr version: killing to defend innocents or self-defense is justified and morally defensible, but can be abused, especially by governments or NGOs that present propaganda as fact.
What about death penalty? Interesting issue for sure. What to do with those who kill others? By killing someone else without justification, they have forfeited their rights in a society to live peaceable. Would life in prison be a better solution? Maybe. But maybe the Hammurabi Code got it right back thousands of years ago. Certain crimes require certain punishments. I am pretty OK with the death penalty, but I understand why others are not.
What about nonjudicial killings, like government operatives involved in espionage or drone strikes? Here is Obama on this topic:
Obama Said He's 'Really Good At Killing People' - Business Insider
...According to the new book “Double Down,†in which journalists Mark Halperin and John Heilemann chronicle the 2012 presidential election, President Barack Obama told his aides that he’s “really good at killing people†while discussing drone strikes.
Thousands of people were killed in the name of the people of the United States of America last year alone, many of which were innocents, none in a declared war.
I really don't know how justified such behavior is. I question strongly attacking any civilians in a war, including things like firebombing Dresden and Tokyo and dropping the a-bomb on Hiroshima. Drone striking a wedding seems very unjustified.
Now, non-direct killing seems also extremely morally questionable. In WWI, the British enacted an illegal starvation blockage of Germany and kept it going after the end of the war. It resulted in hundreds of thousands of deaths. It is not morally justified.
Ultimately, there always be justifications for killing. Many will be completely immoral, but may be understandable. John Wilkes Booth felt justified in assassinating Lincoln, after all. Brutus and the rest of the assassins felt justified in killing Julius Caesar. In both cases, I believe the murders were immoral.