In Silence Of The Lambs, Hannibal Lecter tells Clarice Starling that Our Billy wasn't born a criminal, Clarice. He was made one through years of systematic abuse. Billy hates his own identity, and as far as I remember such abuses were not discussed in the movies. what do we know about that?
Leaving aside that Lecter is a famous psychiatrist and would have had experience of such a common psychiatric disorder, the fact is that Lecter knows who Buffalo Bill is.
This is not really covered well in the movie for, I suspect, time reasons but, in the novel, it is made clear that Lecter had a patient, Benjamin Raspail, who was involved with Jame Gumb and Rapsail spoke about him extensively in treatment sessions.
In fact, the head found in the storage garage belonged to Klaus, Raspial's lover, who was killed by Gumb.
In the novel, Raspail explains this to Lecter (just before Lecter kills Raspail)...
“Jame found the little beach house Klaus and I were using, and when I came home from rehearsal, there he was. But I didn’t see Klaus. Klaus wasn’t there. I said where’s Klaus and he said swimming. I knew that was a lie, Klaus never swam, the Pacific’s much too crashy-bangy. And when I opened the refrigerator, well, you know what I found. Klaus’s head looking out from behind the orange juice. Jame had made himself an apron too, you know, from Klaus, and he put it on and asked me how I liked him now. I know you must be appalled that I’d ever have anything else to do with Jame—he was even more unstable when you met him, I think he was just astounded that you weren’t afraid of him.”
Silence of the Lambs - Thomas Harris