"Deathgrip" (DETECTIVE COMICS #524, 1983)
After an encounter with a new villain known as "The Squid," Batman barely escapes with his life. But the Squid turns out to not be the real new threat to worry about in Gotham, as one of his associates, a reptilian-skinned man known simply as Killer Croc, is the real menace. In this first appearance, he takes out the Squid, emerging as a more dominant criminal in the Gotham underworld, and soon, one of Batman's greatest villains.
"Crocodile Tears" (DETECTIVE COMICS #660, 1993)
After already having broken both of his arms by the musclebound criminal known as Bane, Croc is dead set on vengeance. After Bane kidnaps the Tim Drake version of Robin and holds him prisoner in the sewers of Gotham during the KNIGHTFALL saga, he is ambushed by Croc, seeking revenge. The two have a massive brawl which brings down the sewer walls around them, and which leaves them both for dead, although of course both survive and return to plague the city.
"Swamp Things" (BATMAN #521-522, 1995)
When Killer Croc escapes from Arkham Asylum, he finds that his consciousness is devolving into a more animalistic, primal state of being. When a mysterious force guides him to look for something called the "wet dark," Croc finds himself compelled to go to the Louisiana bayou, where he encounters Swamp Thing. The Elemental being then informs Batman that Croc has devolved into becoming a part of what he calls "the Green," and must remain there with him.
"Hush: The Ransom" (BATMAN #608, 2002)
At the beginning of the "Hush" storyline, Batman faces off against Croc, who is more reptilian in appearance than ever before. Having kidnapped a young boy, Batman realizes that kidnapping was never Croc's M.O., and along with his more animal like appearance, realizes someone has been altering and controlling him. It’s revealed that by infecting him with a virus, one which makes him more animal-like, he’s now controlled by someone else: the villain called Hush.
"The Beast Beneath: Croc's Tale of Tears" (DETECTIVE COMICS #810, 2005)
Killer Croc looks for a cure for his more animalistic condition by enlisting Doctor Maria Belleza, who finally manages to create a cure for him. But after injecting Croc with the serum, his body reverts to his more human-like form after just a few minutes. She tries again, but now hungry and enraged, he kills and devours Maria Belleza, fully giving in to his animal side. Although he feels remorse for this act, he knows he is now fully an animal and no longer a man.
"Cherish Is The Word I Use -- To Destroy You!" (RED HOOD AND THE OUTLAWS #3, 2012)
Following the events of 2011's FLASHPOINT, much of DC continuity was reimagined. In one of his earliest appearances in the resulting "New 52" universe, we see that Waylon Jones is a kinder version of Croc in this reality. We see Roy Harper/Arsenal's flashback to a time when he fought Killer Croc, who realizes that the young hero is looking for him to kill him in battle, and is barely putting up a fight. Instead of murdering him, he helps him get his life in order.
"The Black Vault, Part One: I Wanna Be Sedated" (SUICIDE SQUAD #1, 2016)
Most recently, Killer Croc was recruited into Amanda Waller's Suicide Squad. On their first mission, Croc and the Squad embark on a mission to bring back a "cosmic item" from a Russian undersea prison, which turns out be a portal to the Phantom Zone. Managing to fight the Kryptonian General Zod together, he forms a deep bond with his fellow Squad member June Moon, the Enchantress.