I have been trying to find any argument against this statement:
As good and bad can't be objectively defined because everyone has a different vision about what's good and what's bad, it becomes that good and bad things for person A could be bad and good things for person B (good for A is bad for B and vice versa). So good and bad number of definitions are as extensive as population itself.
This implies that what's bad for most of the people (raping, homicide...) could be seen as good in certain circumstances for somebody and think they are doing the correct thing. Then, you can not say that person is "bad" if you could certainly know he considers his acts purely good. All you could say is that he is "bad" the way you consider good and bad things.
For example, imagine that in the year 1944, there is a nazi soldier who really thinks killing Jews is the right thing. Imagine he feels like he's doing the right thing (much as you would if you gave a sandwich to a homeless person). Then that nazi soldier could be a bad person for you, but you couldn't state he is actually bad because he's not conscious about doing anything wrong. Only by changing his vision of morality he could think he had acted bad.