From Halo 3: ODST's audio-files, we have Police Commissioner Kinsler. Over the course of the story told in the various audio-packets you find throughout the city, he tries to capture and rape Sadie multiple times, has the city Superintendent shut down, hindering rescue work and thus likely resulting in hundreds of unnecessary deaths, casually orders the execution of Dr. Endesha (Sadie's father), has his troopsopen fire on a tightly-packed group of civilians who were trying to get to safety in his train car, and other things I can't remember at the moment. The last of those acts gets him killed when Vergil opens the train doors, allowing the angry mob to rush in and literally tear him apart.
There's Prophet of Truth - he's The Caligula who intends to wipe out everything in the universe with Halo. He attempts to commit genocide against the Elites, causes trillions of deaths just so that he won't lose power, and after coldly murdering Miranda Keyes, he forces Johnson to activate the Halos while mocking him and calling humanity weak. Never mind that Miranda was something of a student to Johnson. His death at the hands of the Arbiter is pretty damn satisfying. At least Gravemind gets a point for being a Magnificent Bastard. Also, he knew the Great Journey was a lie, according to the books, and ordered the genocide of humanity anyway in order to get himself named Hierarch.
Haka, from the Halo Legends DVD adaptation, easily qualifies. He has a friend of the then-Arbiter murder the Arbiter's wife, after an off-screen beating that, judging from the injuries, was horribly brutal. He then lures the Arbiter into an ambush to kill him and usurp his clan leadership because the Arbiter wasn't part of the Covenant. Oh, and while most Elites would have had the justification of the Great Journey, believing that only a unified Covenant could reach Paradise, Haka didn't give a damn. To him, the Journey was simply a means to control other Elites and take power.
The Forerunner known as the Master Builder, a.k.a. Faber. Good Lord, he'll utterly tear your "Forerunners were noble heroes" view apart. 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...so he could toss those populations to the Flood and see what happened. Unfortunately, whether he died or not hasn't been confirmed, which hopefully means he'll be in Halo 4 so we can frick him up.
