The Prophet of Truth, Ord Casto, was one of the Big Bads of the Halo series and instigator of the attempted genocide of two species. After finding out that humans were the descendants of Forerunners, thus violating Covenant doctrine, Truth, along with his cohorts Mercy and Regret, decided to wipe out humanity rather than risk their species losing power. While originally more concerned with personal power instead of the Covenant religion, as time went on Truth became more and more devoted to the idea of procuring godhood for himself through the Great Journey, and took more drastic steps to make it a reality. Believing the Jiralhane (Brutes) to be more vicious and mindlessly devoted pawns than the Sangheili (Elites), Truth secretly ordered the genocide of the Sangheili race after ensuring the Jiralhane supplanted them. This triggered the Covenant Civil War. Cold-blooded and ruthless, Truth felt no loyalty or affection for his fellow Prophets. Truth prevented Covenant forces from saving Regret in order to use Regret's death for political gain, and allowed Mercy to die by the parasitic Flood for no reason at all. Even when having evidence that the Halo Rings were not devices to procure godhood but were instead weapons of mass destruction, Truth decided to activate all of them anyway, willing to risk the death of all life in the galaxy just to have the chance to become a god.
Police Commissioner Kinsler, a brutal thug and Dirty Cop who took advantage of the chaos caused by the Covenant's invasion of New Mombasa to carry out his crimes without repercussion. The first time Sadie encounters him, Kinsler reveals his intention to rape and kill her. When Sadie escaped from him with the help of Kinsler's driver, Mike Branley, Kinsler swore revenge on both of them. After Vergil, a subroutine of the AI that maintained New Mombasa's infrastructure, prevents Kinsler from killing Branley and recapturing Sadie, Kinsler responds by shutting the AI down. This has the effect of grinding New Mombasa's evacuation to a hault and causing many unnecessary deaths. Kinsler's final attempt to capture Sadie has him threatening to murder her father if Sadie doesn't willingly turn herself in. When Sadie arrives at Kinsler's private train car, Kinsler's having his corrupt officers fire into the crowd of panicked refugees to prevent them from escaping on his train. Once Sadie's in his clutches, Kinsler reveals that he already murdered her father by flooding his lab with argon and then tries to have his way with her again.
Faber, the Forerunner known as the Master Builder. Power hungry, vain, and a horrible racist to other aliens and basically anyone who isn't a builder, he built the Halos basically so he and his pals could be in charge. When the San'Shyuum (Prophets) revolted after they realized the Forerunners were losing to the Flood, he figured the fair punishment was to use their homeworld as a test subject for his Halos. And when the Forerunner council told him he had to allow the Librarian to use the Halos as preserves for sentient life, he allowed it only so he could toss those populations to the Flood and see what happened.