Various people are shot and killed with brief but bloody results.
Numerous rough fist fights break out between characters with some blood. The final fight involves stabbing, and a character being stuck in a helicopter, which falls out of a building and explodes.
The riot scene is quite violent, with many villains from Batman's 'Rogues Gallery' killing guards, police officers, and fellow inmates. Many people are shot, stabbed, crushed, thrown around, and blown up in this scene, though it isn't all that brutal.
Three characters' heads explode at different points in the film, two with bloody results. Joker then holds the head of one of them and cracks a joke.
A man pulls another man into a bush, and we hear the victim's neck snapping.
Killer Frost can freeze her victims before smashing them to pieces.
The Joker shoots and kills several people.
Batman breaks several bones during the opening fight scene.
Quite a bit of violence for a PG-13 movie, though not the worst of the DC Animated Features.