THUG 1: You shouldn't have turned the gun on that kid.
Things would have been different for Jack Napier if he had turned the gun on the kid and fired. For him, being merciful in this rare instance totally backfired.
HARVEY (on TV): Together, we can make this city safe for decent people.
JACK (watching TV): Decent people shouldn't live here. They'd be happier someplace else.
Jack, as part of Grissom's crime gang, takes pride in making Gotham a city of criminals. But don't criminals need decent people to prey on? If everyone were a criminal, would there be any crime?
ECKHARDT: You mean when you run the show? You ain't got no future, Jack.
Jack has aspirations to be the big boss of Gotham City, but Grissom's other lackeys don't want it to happen. It seems that being a criminal is a lot like politics. You have to make sure you have the right people on your good side, but there may come a day where the right people become the wrong people.
JOKER: Can somebody tell me what kind of a world we live in where a man dressed up as a bat gets all of my press? This town needs an enema!
To Joker, Batman is a criminal, taking away from him what he really wants: attention.
KNOX: His parents were murdered in that alley. […] Poor kid watched the whole thing happen in front of him. […] What do you suppose something like this does to a kid?
Criminality is a cycle. Often, people who have had crimes committed against them commit crimes. Bruce Wayne becomes Batman, a vigilante, which is technically illegal. And he grows up to kill the Joker, which, while appreciated by the citizens of Gotham, is also technically a crime.