What is the Joker’s real name? How did he become so insane?
Why he became so insane is irrelevant. It would just take up space to go into that, and it’s not the primary focus of this answer.
I think his real name is Roland Daggett. The Joker makes fun of him right before poisoning his drink at Gotham City’s Christmas tree lighting ceremony in “Batman Returns”. It’s worth noting that in this scene, the Joker seemed to know who had paid him to do it as well. So while we don’t know why he turned insane, we can safely say he was always a calculating villain with an agenda rather than a madman incapable of thought like most comics make him out to be – Batman can attest to that better than anyone after all these years battling with him!