Dark Eldar - They do drink the souls of any being they can grab their hands on, and their entire society is based on deception, violence, torture, pain and treachery. They are my #1 in my opinion (especially since they had the choice to give up their decadent ways like their Craftworld Cousins did).
Chaos Space Marines - Most hated, treacherous force in the galaxy (well according to the Imperium). Also one of the most dangerous, especially if united. Depending on which legion (although being caught by any of them is bad enough), if you get into their clutches you can expect a pritty awful end - For example in the extreme of the 4 gods
World Eaters - Some incredibly violent, gory and painful end full of rage and slaughter.
Emporer's Children - An EVEN MORE incredibly violent, but on the other hand maybe even "pleasurable" end than that being in the World Eater's clutches, but if Fabius Bile was on the side lines expect the "pleasure" side to evaporate quicker than water on a solar panel.
Plague Marines/Death Guard -Awful, slow diseased descent into death and then into a Plague Zombie. Probably the worst way to go being that your body would rot while you were still alive, not to mention you are being tortured at the same time in all likelyhood.
Thousand Sons - Probably just an endless tormented Spawn at the whim of a sorcerer, with a sutible "Tzentch(sp?) esque demenour. Eternity as a spawn sure would suck, and be incredibly painful to keep changing.
Not to mention Word Bearers torturing to please all the Chaos Powers in turn, or Black Legion just transforming you into a slave wher the slightest trangression involves an excruciating and violent death.
The Chaos Gods - Although considerd evil in many ways, a lot of fluff states some groups as interpriting them as less than what the Traitor Legions make them out to be. However, Slaanesh does eat the Eldar Souls, Khorne exists for nothing except violence and rage, Nurgle exists just to see things and races disentegrate painfully and Tzeentch just decides to do what he likes, with no sense of right and wrong. The counter argument of course could be made that They all do just what comes naturally, as they are the living embodiment that comes from the beings they are for lack of a better word "dreamed" up by.
The C'tan - Not much needed here considering the total OVER THE TOP amount of fluff on them, but 4 super powerful beings who dont really need much more power just deciding to wipe the Galaxy of all life because they can is a pritty nasty side. Plus the fact that they destroyed most of the Necrontyr before they accepted them shows that even their servants suffered horrendously before they were included (if you can call it that, more like enslaved) to their will.
(I did not involved the Tyranids simply because they dont seem to be aware of what they are doing is "right or wrong" and for the same reason the Orks, as they also don't reall know what is going on and have no real concept of "right and wrong" as we would see it).
If there is another race that you would deem "evil" that I have missed, feel free to add it in and explain your reason why as to who is the baddest race in 40k