Leaving aside Joker, who embodied all of Gotham's madness and darkness-
Two-Face, Harvey Dent, had believed in justice, only to suffer betrayal and disfigurement. Once his convictions crumbled, he became a slave to chance and revenge;
Penguin, Oswald Chesterfield Cobblepot, had faced years of discrimination and rejection because of his appearance, using violence and power to fill the void left by his inferiority complex and longing to be loved;
Riddler, Edward Nygma, had seen his intelligence go unrecognized and his existence ignored, obsessively using riddles and crime to prove he was one of a kind;
Poison Ivy, Pamela Lillian Isley, had been an innocent girl until malicious experiments destroyed her sympathy for humanity. Hating humans for destroying nature and harming plants and the weak, she turned to radical environmentalism and anti-human ecological terror;
Mr. Freeze, Victor Fries, had been the victim of a laboratory accident. To save his beloved wife, he froze his conscience and emotions for love;
Destroyer, Bane, had been born in a hellish prison and shaped by violence and suffering, wanting only to destroy others to prove his own strength;
Scarecrow, Jonathan Crane, had been a victim of domestic abuse, his childhood and academic career both ruled by fear. He turned to studying fear and using it to control and torment others;
Black Mask, Roman Sionis, had endured a cold, heartless family, pressure, failure, and self-destruction. After putting on his mask, he abandoned his identity completely and became a gangster driven by pure brutality;
Hush, Thomas Elliot, had envied Bruce Wayne's life and family bonds, meticulously plotting his parents' deaths and attempting to "copy" everything Bruce had, only to lose himself in the process...
Beyond them, there were more villains who were slightly less famous but every bit as troublesome-Clayface, Ventriloquist, Calendar Man, Bronze Tiger, Professor Pyg,