If your life is threatened by another person because of an unknown reason that is probably not your fault, seeing as the other person is intent on ending your life for their personal goals, I believe it is a right moment to take theirs. Your life might not be worth as much as theirs, but the fact that they're trying to end yours shows that they're capable of doing that to anyone. If a wife sends a hitman to kill her husband in order to gain life insurance money and take his possessions as well as probably begin a new love life with another person, and her husband gains an edge miraculously in the struggle, it's kill-or-die, and the husband has done no wrong to have his life taken away (just a hypothetical example).
It really depends on the motives behind the killing, and how innocent the one threatened is. So many situations may end up being called self-defense, such as you insulting someone, they try to kill you for disrespecting them, then you kill them. But if you do NOTHING to deserve the attack, then you have every right to take theirs before they take yours. Why would you have to die, you've made no crimes! The other person is the one committing a crime, probably mugging you randomly at night and already stabbed you once so you wouldn't tell the police, but still wasn't sure you'd keep your mouth shut.
In war, you're fighting to survive, as well as prove a point. One country threatens the other, if the threat is a serious threat, then there's no reason they shouldn't prevent the casualties that might happen... But once again, depends on the severity of the threat, and the innocence of the threatened. If the other country promises a nuke after a month, obviously the threatened will not sit back and hope it's lying or resort to diplomacy since the other country's mind is made up and stubborn. There's too many lives on the line that literally have NOTHING to do with the conflicting governments!
Sure, it'd be smarter to sit down with the opposing side in a battle and talk it out diplomatically, in a pacifistic way. And it might work if the problem didn't pose a big enough threat... But, like I've said in the past paragraphs, the option to kill shines brighter the more severe the problem between the two opposing sides, and it's only an option if you're not responsible for your position. The one side to have the need to end it before it's too late will make the move, and there's no way the other side can predict it and probably was sitting down like a diplomat. An easy answer? Yes it was! The right answer? it depends on what was at stake.
tl;dr: It all depends on the severity of the matter at hand. It may never be the best choice in minor struggles, but when either other lives are at stake, or a country's welfare or freedom, then yes, it's worth it. Any other problems can be resolved by sitting down and reaching even grounds to understand each other and solve it with no conflict. Sometimes... Killing can be seen as the best option, the fastest option, or even the only option, when in fact it shouldn't even be considered as an option in the majority of scenarios..
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I'm religious, but I'm also realistic. I might never kill a man in my life, but that doesn't mean that others won't either. Might as well give my input according to how I feel.