Senator Decimus is, for the most part, a rich jerk. It's not exactly his fault, as he's a product of his times. He certainly lacks the scheming and ambition of his fellow senator Ratbat, but like most members of the Senate from his time period, he's simply lost touch with anything but maintaining the comfortable lifestyle to which he's become accustomed. In another era, he'd simply be a harmless dignitary, but with the class struggle erupting from his continued automation of the asteroid energon mines, he's sitting on a powder-keg just waiting to blow.
IDW comics continuity
A routine procedural appearance turned dangerous when Decimus announced the facility re-designation terms for Mining Outpost C-12. An energy-pick flung by disgruntled miner Megatron nearly cost him an arm, and the subsequent mutiny on the penal transport left him stranded in space and in need of rescue by Autobot Security Services.
Of course, one of the cardinal rules in pre-war Cybertron is that the energon must flow. Thus, Decimus's valiant efforts to keep the supply up (albeit mostly in senate hands) earned him a fetching statue of his likeness and a parade in his honor. It would have been quite nice if he hadn't been abducted by the Seekers during this parade and taken straight to Megatron.
Decimus was rescued by Sentinel Prime and his forces later and appeared as one of Kaon's council members during Starscream's appeal. He was killed by Starscream after the Seeker delivered his message.