The Dark Knight (2008)
The Joker explained The Batman what is Madness.
Batman: You’ll be in a padded cell forever.
The Joker: Maybe we can share one. You know, they’ll be doubling up, the rate this city’s inhabitants are losing their minds.
Batman: This city just showed you that it’s full of people ready to believe in good.
The Joker: Until their spirit breaks completely. Until they get a good look at the real Harvey Dent, and all the heroic things he’s done. You didn’t think I’d risk losing the battle for Gotham’s soul in a fistfight with you? No. You need an ace in the hole. Mine’s Harvey.
Batman: What did you do?
The Joker: I took Gotham’s white knight and I brought him down to our level. It wasn’t hard. You see, madness, as you know, is like gravity. All it takes is a little push!
[the Joker laughs hysterically as Batman races off and the cops come to take the Joker into custody]