The soul can change, therefore the category of which they belong as well. I agree with your questioning the existence of good and evil, it's a very good point indeed that I can't argue with.
Strangely though this is concept, an obscured idea, just as good and evil. They do exist, whether you say you don't believe in them or not because they are merely thought patterns, something put onto paper; although not physically there they do exist.
Just like physically shape-shifting, it doesn't happen but the idea of it exists and it can be defined in a sense.
Good and evil regardless of their physical value, can be put into words-- right or wrong-- wrongly right, or rightfully wrong?
It doesn't matter how categorized, everything can be done so.
Although I do agree with you. 
