I have to respectfully disagree with you here, Malte. I think giving a villain of this kind a background runs too close to make them too human/sympathetic and less menacing. Norman Bates of Alfred Hitchcock's classic "Psycho" is a perfect example of this. In the original version of the film, he was never given a proper diagnosis at the end, like in the theatrical version. So why was the Doctor included to tell us just why the heck Bates is so nutty? The producers said that leaving Bates' psychosis a mystery made the film too unsettling. In a nutshell, they made Hitchcock give Bates a more explained to motive to make him less scary. And yet, Mr. Bates is quite able to look and behave normally to his customers, yet he is completely insane.
The villain from "Disturbia" works the same way, only this time the filmmaker was allowed to not give him a proper background or motive. The villain in "Disturbia" is a serial killer who looks and acts normal on the surface, but unneath lies the mind of a maniac. We never find out why, because its not important. This is a lesson the filmmakers of "Hannibal Rising" did not take to heart, unfortunately.
As for your remark about how one would need to have a certain level of intelligent to fool everyone. I agree, but it doesn't work against my concept. We've seen the type in real life. There have been some maniacal killers in real life that we figure out even after we caught them. A lot of questions were left unanswered, and these are the most famous killers. In fact, arguably, the most famous serial killers in history is Jack and Ripper and the Zodiac Killer, and we still don't even know who the heck they were or why they did it, because they were never caught.
Now, doing it your way by giving the villain a background would probably make the villain function better as a character, but it would take from his ability to function of this kind of villain. In terms of a villain who acts out of insanity, instead of more logical motives, the more unanswered questions that surround them, the more daunting they seem. I think its safe to say that it is no coincidence that the most remembered killers are still make us raise an eyebrow, and that the most effective crazy villains have no background to speak of..
I like the scene you came up with. That would be a clever way for the villain to cover his trail, for a time, anyway.