I was just watching it too(assuming you were watching it on tv as well) i was wondering the same thing. Since he makes up many different stories, no one relly knows. And there probably isnt an answr. But lets go with he went insane and did it himself in order to be callle the joker because he liked the name
According to the story,He had a father who was a very heavy drinker.And one day he was way off,he was also angry that his wife was defending herself with a knife so he took it from her,came to the boy and said,"Lets put a smile on that face!"While it was in his mouth.Slash!