Backstory to fit TMNT 2012:
King Komodo was already in NYC, living with his half-brother Go-Komodo, and a bunch of other monitor lizards like himself. Go-Komodo was a human, while KK was just a regular komodo at the time. Go-Komodo was of Japanese origin and had already learned how to fight. He lived in a wealthy part of NYC as well, so he had a relatively untouched life. Go-Komodo was also very interested in mutagen, so he had some synthetic mutagen made for him. Go-Komodo was looking to turn into a himself into a super-human being, whatever it may be, and fight with supremacy.
Go-Komodo had upset a group he had done some business with, and they came in one night to ransack his home, and find that mutagen. During the fighting, KK was somehow accidentally injected with the synthetic mutagen, then in that moment turning from an average komodo to a vicious mutant that achieved sentience. Along with a "human" sense, he had improved venom and the ability to control reptiles with his mind. The other lizards were either killed by the ransackers, or lived but untouched by the mutagen. Living within a very combat-oriented household, he's picked up on some moves and techniques over the years.
Go-Komodo decided to make some use of him, and after seeing what synthetic mutagen did to KK, he wanted the real stuff. The real stuff belonged to the Kraang.
King Komodo would often go out and look for dinner around the city, meaning eating people. He brought his monitor lizard brothers with him.
A night after KK's regular explorations of NYC, he and his "brother" lizards brought in a mutagen canister that was scattered after an airship accident, and presented it to Go-Komodo. Go-Komodo then ordered KK to find more of these, and bring them back.
During one of these nights, the turtles were also looking for the loose canisters. They did hear about-- and find -- some eaten humans, and they were often dumped in the sewers. Donnie had his little radar thingy going, as usual, to find the missing mutagen. KK and gang found a canister at the same time the turtles did. There was a standoff, then a skirmish, then a full blown fight. A lot of KK's brothers were killed in the fight, and he himself suffered some pretty bad injuries. He snatched the canister and escaped.
He was not, however, happy with the fact his brothers were killed. He returned the mutagen, injected a little more to heal up the wounds, and now hunts the city for those turtles to seek revenge.