Welcome to our visitors.
I want to point out so that no debate begins, this is The Ancient Way, which is the Eastern Orthodox forum. We are glad to have visitors.
But because all human beings are created in the image of God, it is always a sin to destroy that image. It might be LESS sinful than failing to prevent that person from carrying out some terrible evil, so it could be the right thing to do in a given circumstance. Or at least a lesser sin. But it is always something to grieve over.
It is even possible that someone who is forced to kill another person (even to defend themselves or innocent people) can damage themselves spiritually by convincing themselves there was no sin so that they can feel justified. The spirit may recognize the sin and NEED to grieve and repent. Repentance is healing. Nothing is gained by denying sin just for the sake of being comfortable.
But in any case, God isn't looking to condemn on technicalities. The whole purpose of salvation is to heal our fallen nature and restore us to our full humanity in the image and likeness of God, in perfect communion with Him eternally. And denying sin as sin that might prevent the opportunity to repent doesn't help that process.
But it may indeed be a NECESSARY and/or unavoidable sin, in some circumstances. The world after all is terribly fallen and broken.