Sentinel was once a trusted ally to the Primes, until the Quintessons attacked them and Sentinel betrayed them, killed almost every single one of them just to have all the power of Cybertron for himself. He even took Megatronus Prime's cog as his own! Before most of the Autobots came online, he took out their cogs, claimed that the energon stopped flowing in Cybertron, and forced them to mine for energon, not knowing he was giving the energon to the Quintessons!