Kellman visited Neo Phobos and was paid in gold by a Vogan faction led by Vorus to ally himself with a group of Cybermen and lure them to Voga - as he learnt the asteroid was called - so they could be destroyed by a rocket.
Kellman obeyed the Cybermen's instructions to use the Cybermats to infect the Beacon's crew with a plague, which killed off everybody except Commander Stevenson and Lester. The Fourth Doctor exposed him as the perpetrator and he was taken prisoner. When the Cybermen boarded the ship, he was allowed to transmat to Voga where he tried to find Vorus.
However, civil war had broken out and he was taken prisoner by a faction opposed to Vorus. He explained to Chief Councillor Tyrum what Vorus' plan entailed and went with Harry Sullivan to try to stop the Cybermen from planting their bombs. As he was climbing, Harry accidentally caused a rock fall that killed Kellman. (TV: Revenge of the Cybermen)