Are evil people 'normal'? Are they 'banal' (not the same thing of course)? The key question, for me at least, is 'are they like us?'.
Of course 'like us' is a vague term. For example, perhaps evil people are just like us in their innate, neural, genetic, features, but they have been warped by circumstance to be unlike us. Perhaps, given the same lifelong experiences as them, we would have turned out like them.
A stronger argument would be that all that seperates us is short term opportunity. If you or I were put into a prison camp as a guard (it is argued), we would quickly degenerate into monsters. I know a lot of people think this hard thesis is true. But I don't. Opinions welcome.