Good and evil is definitely perception.
The society sees an act as a norm, it is acceptable, therefore, not evil. If the society sees an act of goodness as the opposite, darn right it is not good.
Good and evil exist only on the conscience of an individual. If you know and ACCEPT that a particular act is evil, and yet you do it, you're committing an evil act. So do vice-versa.
The paradoxical dilemma only occurs when two culture, as in two societies with ultimately different in mentality, clash. Hitler thinks what he did was right, ethic cleansing, by religion, environment, education and acceptance of his society, therefore, to him and his society, he was not evil, but more of the opposite. The rest of the world thought differently. That proved that good and evil is, also, relative.