How we cite our quotes: All quotations are from Batman.
Quote #4
JOKER: I've been dead once already. It's very liberating. […] Jack is dead, my friend. You can call me... Joker. And as you can see... I'm a lot happier.
After death, Jack Napier is reborn as Joker. He says he's a lot happier, and that's true. After he is dead, he has very little cares in the world. He has the freedom to do whatever he wants, no matter the cost to society. That would make any homicidal maniac happy.