Doctor Sivana used to live on Venus.
Sivana used to be a kind and optimistic young scientist who only wished to make the world a better place. But his new ideas and revolutionary inventions and theories got rejected over and over again by cynical politicians and crony industrialists who liked the world just the way it was. Growing bitter, Sivana did what anyone else in his shoes would have done.
He build a space-rocket and flew to Venus with his children. And he stayed on the planet until his kids had reached adulthood, then went back to Earth in order to conquer it.
Harley Quinn is super-human.
After almost getting killed by her darling Pumpkin, a badly wounded Harley was found by Poison Ivy who fixed her up AND gave her superpowers as well. She is immune to poison and has slightly enhanced strength and agility.
Chameleon is in love with Peter Parker.
Chameleon once found out who Spidy really was and took on the appearance of Peter Parker as a way of ruining his enemy's life. But after walking in Pete's shoes, experiencing his life, Chameleon grew more and more obsessed, NOT with Spider-Man, but the man under the mask.
In the end, Chameleon realized that he was madly in love with Peter.
Red Skull became a monster after killing a girl he loved.
Back when Johann Schmidt was a young, "troubled" man, he fell in love with the daughter of a shopkeeper he worked for. Her name was Esther, and she was the first person in his young life who had been kind to him. He wanted to confess his love for her, but being the "troubled" young man he was, he then instead tried to force himself on her, then murdered her.
He felt great after this horrible act and realized how much he liked hurting and killing.
Joker sold his soul to Neron.
In case you do not know, Neron is to DC what Mephisto is to Marvel. He makes deals in exchange for souls, usually villains who gets a powerboost or something else they need.
Joker sold his soul to Neron in exchange for a box of Cuban cigars.
SURE! WHY NOT?!