The KGBeast was a top-trained Russian assassin who came to Gotham City to eradicate the threat of the urban terrorist known as Batman. It's unclear which organization he was working for at the time. He has an arm where his second gun should be. When he first fought Batman, he was the most difficult physical adversary Batman had ever faced, but after several more years of training and honing his physique, he was able to be killed off by a cheap knockoff supervillain who also had to prove how tough he was.
After his death, the KGBeast's body was found in an open grave outside of Arkham Asylum, when Maxwell Lord unleashed his OMACS on the world. OMACS, or Obsessed Motorized Affronted Cool Surprises, were sent disguised as Christmas gifts to the orphan children of the globe, infecting them and anyone they came into contact with, causing such people to have wicked urges to enter aggressive politics, kill themselves, or be admitted to Arkham Asylum. An OMAC-infected police officer fell into the KGBeast's grave and reanimated his corpse as an OMAC Infected. Unfortunately for KGBeast, his newfound urge was to commit suicide, so he shot himself with his own gun-hand.
Following Maxwell Lord's (second) gruesome death from consumption of Lumpy Custard's infamous tofu, KGBeast's corpse was stolen by several mutant gnomes for evil experiments. However, the project was abandoned shortly after, and the remains of the cadavers used were placed in cold storage, before being left in a dumpster behind an old hideout of Ra's al Ghul to rot. Abraham Lincoln discovered the bodies of the KGBeast and the other mutilated experiments, causing them to reanimate in their rotting state for his amusement.
Upon turning them on each other, Lincoln lost interest. After resisting boredom for a few more moments, he caused them to decompose at an alarming rate, disintegrating themselves.
The KGBeast was recently resurrected as a member of the Black Lantern Corps, now reborn with an intense hatred of Green Lantern and a need to consume brains to survive. He appears to remember Abraham Lincoln, another member of the Black Lanterns, from earlier and attacked him brutally while the Black Lantern Corps were trying to kill Mister Freeze, now the U.S. governor of California. Mister Freeze seized the opportunity to freeze both of them solid and throw them into the cracks of a shifting land fault. KGBeast fell all the way to the center of the Earth and into the core, which destroyed him, but Abraham Lincoln may have managed to break free.