Thrull was the planet that Galvatron ended up on after being thrown out of Unicrons body.
Spending such a long time buried in one of Thrull's Plasma Pools severly damaged Galvatrons mind, driving him stark-raving mad.
After violently attacking his own troops for attempting to steal him away from his "kingdom", Galvatron left Thrull. But not before Galvatron destroyed the planet for no reason at all while cackling madly.
Galvatron's pretty crazy.