That would also require that you're always able to find someone like that before you starve to death. Pessimistically, its viable, especially if you don't mind breaking into prisons. On the more realistic side though, your character's judgement on whether a person deserves to die isn't always going to be accurate. I don't recall any lore about vampires being able to discern a person's true heart and information you hear from other people and/or from your observations can be flawed, especially if time is an issue. Then we get into the moral ambiguity of vigilante justice outside of self-defense or direct defense of others.
