Most people try to be good within their own value system. This value system relies on what they know, and have been teached. Some nazis executed Jews and were proud of it because they genuinely thought it was a good thing to do.
One of the hardest parts of ethics is to find a sufficient subset of values that are actually shared by humans universally. I don't think such a thing exists, as our cultures and psychologies vary greatly in that regard. Even talking about killing is extremely complex as things like self-defense, war (or even justice in some backwards cultures mkay) can change our perception of the problem.