Pretty much any race from the WH40k universe, discounting the Tau, Eldar and various 'good' human factions (Imperial guard, Space marines, Sisters of battle, Inquisitors...)
So that leaves us with Dark Eldar ("Hey, I like the way you scream! I'll make you scream for the next few dedaces" - so after a few decades of torture, you, assuming you still live, can't tell the difference between pleasure and pain as you serve your masters, who torture you even more just for the kicks.)
The Ork ("Wesa smash you good! WAAAAGH!" - que a rush of an unstoppable horde who can't really be stamped out without a full-scale planetary bombardment. A single Ork spore is enough to grow them all back and their technology is genetically programmed into them. Their minds also make it work: every ork knows red vehicles go faster, so painting a tank red does make it go faster. If a weapon looks 'good an proper Orky' it will work, no matter what the laws of physics would say, as long as it's in Ork hands.)
Chaos ("SAnITy iS FOr tHe WEaK! Blood for the Blood God!" etc. Their 4 gods are also very real and the home of Chaos is pretty much comparable to medieval idea of hell.)
Necrons ("The Living have come. We shall cleanse this world in the name of the Star Gods" Who literally eat stars, but happen to like souls a bit more, for the taste)
Tyranids (Yeah... Xenomorphs have nothing on tyranids. Their heavier fighters would eat entire xenomorph lairs for breakfast with a sidedish of a few Predators... oh and they are all psychic. Their scouting fleets alone have been declared the greatest threat to life within the galaxy. The main invasion fleet is still far away...)