But consequentialism/utilitarianism [1] isn't the only form of moral reasoning. By most forms of deontological [2] morality, saving someone's life is a good act by itself, even if the fella goes on to become a murderer later.
The first one. If you are trying to prevent loss of life, that is admirable. Even if the people you save choose to commit violence later, that is on them; the responsibility is not on everyone who had previously helped or healed a (potentially) violent actor.
But you’re saving a soldier, in an active warzone. You can’t pretend to be naive and say maybe they’re going to have a change of heart and become a flower farmer. By saving that life, you’re executing the future victims of that killer/soldier. That you don’t actually pull the trigger on those victims is neither here nor there.