In that past, a version of Lazarus from an antimatter universe had visited this universe. Unable to bear the thought of a duplicate, Lazarus became insane, and obsessed with destroying this anti-Lazarus.
Assigned to investigate, Captain Kirk and the USS Enterprise confronted Lazarus several times, attempting to learn the truth. Lazarus referred to his counterpart as a "beast" and claimed he had destroyed Lazarus' entire civilization. Kirk and Spock soon realized there were two versions of Lazarus.
In a final confrontation, Kirk was accidentally transported to the antimatter universe, where anti-Lazarus revealed the whole story. He confirmed that if Lazarus succeeded in crossing over to the antimatter universe, and there confronted anti-Lazarus, the result would be the destruction of both universes.
Together, Kirk and anti-Lazarus forced Lazarus into the negative magnetic corridor that connected the two universes. Kirk then destroyed Lazarus' spaceship, which also destroyed anti-Lazarus' ship, sealing the corridor, saving the universes – and trapping both versions of Lazarus together for the rest of time. (TOS: "The Alternative Factor")