I believe his mother put the wig on him, to fool the neighbors into thinking that she only had a little girl, so that the Bonaparte would not find her.

with all the atrocious things he had done, from a series of murders of his foster parents, from manipulating some peoples to do murder for him, from orchestrating a massacres be it underground business, towns people and a whole institute to kill each other(he's 10 at that time) and making a lot of people to commit suicide including kids and thats before making them feel sorry for even existing. brainwashing a lot of kids to breaking point. and some other horrifying stuffs he's done that isn't mentioned in the manga.

when he was shot, they still saved him. After all those events were finished none of his crimes, ever did come to light, with no charges at all, they never told anything about what johan did and just pressed all the charges on johan's dead lacky roberto. with johan just sleeping in a hospital waiting to recover.

and so in the last part of the story it was hinted that johan escaped the hospital. no one know if he'll go back to his evil deeds again so..