When it comes to joker you can't compare him to a serial killer. A serial killer (and mostly all other crazy murdering people) live in hiding. Many with what looks like "normal" lives. Hitler craved power, order and control, Joker craved chaos. To even compare Hitler to Joker shows a lack of understanding when it comes to the joker.
He walks around with scars and paint smuthered all over his face, easily managing to set up one scheme after another without Batman and the police force getting any tip. Everything about the joker is absurd and not grounded in realism. He does not pretend to be something else and does not play along to get people to follow him. So no way he would get so many followers. Not grounded in realism. He wandered around as himself completely only wanting chaos. Such a character is not grounded in reality, he could not have lasted long before commited.
Nolan chose to not explain him and he is a driving force of evil not at all grounded in reality. This is no Hannibal, living like a normal man eating people in his spare time. This is a evil man, not crazy, with an IQ way up there who manages to not only stay hidden but also plan everything down to the precise second and nothing ever goes wrong (before the end, or did it?).
He is so unreal that even Bruce Wayne, a man who spent years to find understanding of the criminal minds, is fooled and not before the end he kinda got to understand the crazy mind of the joker.
A serial killer craves for the kill, a pedophile wants to get it off, a gangster wants money. Hitler wantet power and control. Joker wants nothing of the kind and he did not pretend to be something else. Still he managed to walk around in at least a year, he was the most recognizable of all but he still managed to keep hidden (Batman easily found everyone else he was looking for).
All about the Joker, what he did, what drives him, how he managed to do what he did was not grounded in any kind of realism whatsoever. That's what makes him so scary, because people fear what they do not understand. And The Joker is no character to understand, because he is not grounded in reality. He is grounded in your worst nightmare.