Perhaps take as a theme "evil is error". People aren't born bad, they just make mistakes, and act out of weakness, until they end up in places where doing terrible things to other people seems like a good idea. Hence Diana trying her best to get people second chances, show them the error of their ways.
Of course, the other side of the coin could be, this is what Diana believes. Doesn't mean its *always* true- and how she deals with someone who doesn't appear to be motivated by error and weakness could be a thing. I'm not sure I really want to see Dr Psycho, though, and he's the obvious choice for this role.