It isn't as simple as that. They represent emotional aspects driven to their extremes. The extremes are evil to be sure, but the same levels of dark cannot be applied to every step of the path that leads there.
Take Kharn for example. He is a mad butcherer and slaughterer of men. But to begin with he was an honourable warrior who even meditated upon the Eightfold Path. If you don't know what that is, look it up. :wink:
Maybe your looking at this to complicated. These beings were manifested from specific emotions of warp sensitive beings (not just the humans,the eldear were largely responsible for Slaanesh).Because of the narrow concepts they were spawned from, they know only those emotions and seek to perpetuate them and themselves without restraint or intellect. The perfect example is Tzeentch. Who in the Imperium can match wits with him? Answer is no one.He schemes for the sake of scheming. He mutates for the sake of mutations as there is no final form.he manipulates for the sake of maniputlation. Just as his brothers follow their own natures to the end so will he. Khan's past is almost irrelevant. In thrall to the blood god he exists for only 3 things: Burn,Kill,maim.