Capel had spent almost his entirely life around robots, being practically raised by them. He identified with robots more than people, and came to believe that robots should rule over humans. He intended to lead a robot revolution.
He joined the crew of the sandminer Storm Mine 4, where he posed as chief engineer Dask (who he probably had murdered) and reprogrammed the the robot crew of a sandminer to kill the Human crew, at first singly and secretly. Eventually, he ordered them to massacre the survivors, as well as the Doctor and Leela, who had recently come onboard the Storm Mine 4.
The Doctor had Leela release helium into the atmosphere, changing his voice so that SV7 did not recognise him. SV7 then throtlled him to death.