Unlike this movie suggests, psychos ain't complex nor mysterious...
The movie Insomnia with Pacino had one nugget of 100% truth on this:
"You're about as mysterious to me as a backed up toilet is to a plumber"
Can't get more real and direct than that (they're nothing but piles of crap worthy of nothing but a flush)
That's how fascinating/mysterious/intriguing these types are to law enforcement, whom unlike what people may think, DO deal with psychos/sociopaths daily (gang bangers, muggers, wife beaters, etc). They just ain't glamorous enough to make much out of them.
But they are all just as pathetic and insignificant as even the most notorious ones (Bundy, Ridgeway, BTK, etc)
Heck, even Bob Kepler, that writer dude about Ted Bundy, or whomever wrote The Riverman (when Ted offered to help catch the Green River Killer), points out that when Ted began begging for one final stay of execution (his appeals had run out) by appealing to scientific curiosity "For you to truly understand why I liked such things and why I did such things", Kepler 's reply was more or less "unless that helps me catch others like you before they kill people, I really don't care about any of your particulars one way or the other".
I'm not knocking the movie, loved it, but characters like Lecter simply are phonies and conmen (pretending there's anything worthwhile about them), nothing more. Unfortunately idiots like Clarice (and eventually Harris himself whom clearly fell in love with his own creation and the rolling cash it produced) fall for their act.