What they said.

For the first couple of issues, during the flashbacks, they left it ambiguosly whether or not James was actually a killer. Remember that whole sub-plot with Barbara's friend? At first Gordon thought James killed her, then he arrested another guy who confessed, but then you see her key in James collection of "kills". There is a lot of doubt and uncertainty.

Once you get to actually showing James being straight-out evil, he stops being creepy and just begins to be cartoonish. That whole scene between him and Barbara was completely ridiculous.

I'll also agree that his master plan was cartoonish. But that's my point. The less outwardly evil he is, the creepier he is.