The Riddler on the end of the Silver Age in Batman Secret Origins
In which Neil Gaiman shows us what we often tend to forget about the Riddler: He’s insane, yet a genius as well, and the only thing he can think to do is mess with Batman, who he views as his only intellectual equal.
He’s committing crimes because he wants to get caught, to test if his riddles can ever stump Batman.
Of course he can be a killer sometimes, because he’s a sociopath who doesn’t have empathy, but he’s much more likely to kill through an elaborate plan or through proxy than getting his own hands dirty. And nevertheless, to the average person, he would seem sane, aside from his obsession with outsmarting Batman.