So there's this game that has a fairly modest but fanatical following out in the gamerculture called Warhammer 40k. Set in the year 40,000, humanity lives in all corners of the galaxy and has a central government of sorts on the world of Holy Terra. That would be Earth after several apocalypses, failed petty interstellar empires, and colonization attempts. Then something like the year 30,000 or so a hyper-human perfect being stepped up from the population of Earth, unified the techno-barbarian tribes, re established a peace with the Martian colony, and ultimately began the impossible task of establishing a unified Galactic Empire of Humanity. He is known only as the Emperor in all established records of the setting and was both the best and worst thing that ever happened to humanity.
To make a long, epicly styled, and marvelous story short, The Emperor is betrayed by one of his clone-generals and the Empire nearly falls to the forces of Chaos. I do not mean nice, cutesy chaos either. Raw forces of sentient corruption and madness that hunger beyond the walls of spacetime. The kind that make you eat your family for kicks and burn whole worlds to test a new toy.
That's kind of a biased account. The forces of chaos are evil, but (assuming that the party line is true and the emperor is conscious and in control and that the Adeptus haven't simply been running a 10000 year long rehash of Weekend at Bernie's) the Emperor of Mankind is pretty evil to. He's so evil he makes the emperor from Star Wars look like a hippie by comparison.
Yeah, but part of the whole point of the setting is that it's a galaxy where everyone has to be evil. The closest we have to a non-evil faction are the Tau, and they're a) gonna get curbstomped any day now, and b) implied to have some 1984 shit going on themselves. In order of least to most evil, I'd say it goes something like this:
There may be a little quibbling around the middle, but I think that's roughly the order.
Switch Orks and Imperium, and you'd be about right. And then switch Tau and Orks. And then Nids and Chaos. Orks aren't waging a war on anyone because reasons, they just love to fight. They're serious about having a good time. The Emperor is such a fuckball, he deserves to at least be third. Nids are going to destroy all the things, and they're the ones running from something. D'Eldar, though, you got that on the nose. Fuck them.