I think a place to read about serial killers is the John Douglas books. He (John D.) told Thomas Harris about the 2 or 3 serial killers that Harris combined to make the fictional Lecter.

I agree with u/madchad90. It's likely a continuity error that the psychologist says that many or most serial killers start by harming small animals when they're young. Maybe Lecter din't. Maybe it was just his extra finger that caused him to be a serial killer.