There's a scene where a character beats up a seemingly innocent couple; the male half is obviously distressed by the female half's getting hurt, and he in turn is beaten up pretty badly. A gun gets held to his head, and the trigger is pulled. The movie artistically makes it seem like his head popped (like a balloon), when in reality, it didn't. Besides that there's some stabbing, fighting, scary samurai-joker masks, and also a scene where the Joker wipes his hand on his face and smears blood over his lips, creating a bloody grin. Afterwards, he and his partner laugh like deranged people.
Joker as his usual character, is extremely deranged and frightening.