WD fluff describes them as being cursed by Khorne in retaliation for messing up his plans. The only way to alleviate the problem is through stabby killy.
Really interesting fluff twist in that they retreated to the eye of terror to deliberately avoid killing innocents (instead sating their curses' needs on eye of terror denizens), and the story behind DV is them searching out an artefact with which to lift the curse.
Seriously impressed by whoever came up with the concept - pitting them against DAs in turn and you're really left wondering who the good guys are. Proper 40k 'everyone's a bad guy' intro for the new players. Love it.
