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.
[1]: https://en.m.wikipedia.org/wiki/Utilitarianism
[2]: https://plato.stanford.edu/entries/ethics-deontological/