I don't see how that highly intelligent Joker on Dark Night started from this insane person

Rating: 1/10 (7 October 2019)

The Joker on The Dark Night is a highly intelligent person who holds deep conversations with prominent people, capable of planning a complicated bank robbery and was one step ahead of Batman, the police and intelligence. He has a strong personality free from emotionality and he laughed controllably.
I can't see how this mentally unstable person on this Joker becomes that awesome Joker.