Few villains are scarier than those who truly believe they are doing God's work.

Another reason the Joker scares me is that he's &^&#*@&#@*&# crazy. (I don't know what word I just censored, but he is.)
Who else is scary? Religious fanatics.
I suppose I would be scary too, if I thought my only purpose in the world was to

a) kill the Jews
b) kill the gays
c) kill the Americans
d) take over Jerusalem while killing as many Muslims as possible.