Kylgots past is a secret to all, but here is the bits we know.
Before he was selected by Nurgle for his most noble of works, Kylgot was an Empire Statetrooper, in other words, a coward. Constantly finding excuses to move from the front line before battles, karma finally caught up with him, and the infirmary was overrun by the warriors of nurgle. All present were infected by the rot, but only Kylgot survieved.

The warriors carried him off, and once in the waste he was left free to wander. The blessings of Nurgle gave him bigger muscles, but not the brain to use it. He was found by The Seven Samples of Decay as they were travelling from the slaanesh game. They saw a mighty warrior, and brought him along to bolster their ranks, not knowing what a coward they had hired.