Thawn was the controller of the methane refinery on Delta III in the 51st century.
He conspired with Rohm-Dutt to have the native Swampies wiped out because he didn't want them to get in the way of his work. He set up Rohm-Dutt to deliver them weapons; this would allow him to call them a threat, justifying their extermination.
When Kroll appeared, he tried to kill it with an orbital strike. When one of his own men, Dugeen, tried to stop him, Thawn killed him; he justified it to himself that Dugeen must have been a spy from the Sons of Earth. However, the Fourth Doctor cut the power, halting the countdown. The Swampies later attacked the refinery and killed Thawn with a spear. (TV: The Power of Kroll)