In quite a bit of the fluff I've read, particularly the portrayal of him in 'Masque of Vyle', Khaine seems to be the god of violent retribution. There is certainly an implied 'justice' about his actions (and also that he has a hair trigger and goes overboard, so rather like Ares in that regard). Less pure slaughter for the sake of bloodshed and more violence for actual or perceived slights. Which still fits in with the WFB lore because the Dark Elves are still incredibly bitter over the throne being 'stolen' from Malkeith and consider all actions to reclaim what they see as their territory are justified.
Kabal of Venomed Dreams
Whoa whoa whoa, Khorne and the other Chaos Gods have their origins in the War in Heaven (Old Ones edition). While Khorne's the "first" to wake up, and his first daemon prince was human doesn't make him a human baby. Given he's worshipped by myriad races and enjoys the efforts of orks he seems pretty non-specific as to his racial origins. On Khaine as a part of Khorne or as a mask, Khorne and Slaanesh had a throw down over Khaine as they both insist he was of them. He kind of tumbles out of the warp (MAGIC) and turns into millions of heavy metal Abraham Lincolns. So, he's separate.
In the original fluff published in White Dwarf long ago (I think WD127), this was gone into in some detail. Khaine is not Khorne. He was a distinct god of the Eldar and a god of war, rather than violence per se. He embodies all of the Aspects - so the stealthy ambushers that are the Striking Scorpians, the righteous knights that are the Dire Avengers, the raging destruction that is the Fire Dragons, etc. When the Fall of the Eldar happened, the Eldar gods were consumed and subsumed by Slaanesh. This was because the Eldar gods were all essentially Eldar-ish reflections of the Eldar mindset and Slaanesh really underlies all of them being the most perfect reflection of the Eldar. Iirc, it states that Slaanesh "feasted on the Eldar gods" as the race fell, taking each of them in as part of hirself.
But when it came to Khaela Mensha Khaine, Khorne claimed that the bloody handed god belonged to him. You see Khaine was much like Khorne in that he was a god of war and violence, as Khorne was. But Khaine was also like Slaanesh in that he was a creature of passionate enjoyment and obsession. Khorne was murderous violence and blood lust - brutal and hateful. Slaanesh is exquisite refinement and a lover of sensation. Khaine made war a bloody dance of exhilaration and passionate violence. And so Khorne and Slaanesh fought over which embodied Khaine. The psychic battle was titanic. They tore at the Eldar god, first one way and then another, until finally Khaine tore apart and Khaine's fragments, like ashes, fell into the material world, each guided by Khaine to land in the heart of a great Eldar warrior. Each Eldar warrior touched by a fragment absorbed the essence of Khaine and became an Avatar of the god.
Thus both Khorne and Slaanesh were denied their prize and Khaine became the last of the Eldar gods.
Best answer to this was posted in "The Cauldron of Blood", a section of web magazine Druchii Herald (WHFB), where "Khaela Mensha Khaine,Our Lord and Master, Answers Your Pathetic Questions"
Dear Khaine,
Tell them you're not Khorne.
Not yours,
Rork, Lord of Khorne
Pitiful Rork,
It is people like you that make my Cauldron of Blood boil. How dare you even imply that anybody would liken me to that blood-obsessed, mentally unstable moron! You have personally insulted me, as have all of “them” by claiming that “they” would even dare to theorise whether I am Khorne!
This nonsense must stem from your subconscious desire to WANT me to be Khorne. Surely it is your deepest secret wish that your oafish deity actually turns out to be the cunning, suave, subtle, omniscient, omnipotent, mighty, undeniable, unbeatable being that is Khaela Mensha Khaine. All that bloodlust has clearly gone to your head. Or perhaps you are just as much of a simpleton as “they” are. No sane being with at least a sliver of intelligence would want his deity to NOT be me.
However, to put it simply for the simpletons who can’t see the simple answer – I’m not Khorne.
Khaela Mensha “Not-Khorne” Khaine
Sadly, it was illegible from all the blood seeping off Khaine's hand, but such are the wages of war.
Maybe he was dictating.
I demand Secretary of Khaine be a unit type in the next codex.
I like to joke with my opponents that it took every other race working together to create Khorne, Tzeentch, and Nurgle. The Eldar created Slaanesh all by ourselves