The Stalin/chairman Mao equivalent in the communist tau empire. Maybe a cadre fireblade who hates other castes.
You clearly understand nothing about Tau fluff

All the chaos dudes who could potentially see chaos as the good guy- chaos will accept everyone; whatever race, caste, social status, etc. nurgle will save you from diseases. Tzeentch will take you for your intelligence, not your privilege/elitist breeding. Slaanesh... Well... Will sex everyone and everything.
That's how a lot of them see it anyway. Liberation from the lies of the Corpse Emperor.

the guard commander that is happy to win the war with a meat grinder of throwing hundreds of guard lives away for a win. A guard commander who killed his superiors to get promoted by filling dead mans boots.
You just described Commander Chenkov.

The Dark Eldar who just wants to keep his race alive and away from the clutches of chaos.
Asdrubael Vect? Only with more backstabbing...

A mathematician Necron who just wants to see what combat match ups work rather than win or kill everything. The fascination with escalating things to see what happens, and then boiling things down again to fill in his charts... necrons that see humanity as invaders on their tomb world and despoiling what they love rather than the POV of 'foul xenos'
I'm pretty sure Necrons have at least two eccentric general characters. Trazyn, for one, who simply attacks for amusement and to gain interesting trinkets, and that other one who's apparently short-sighted and very friendly to other races.

Marines that enjoy killing a bit too much.
Gabriel Seth?

Tbh, I think I would like a more warmachine style approach. More or less each faction in warmachine/hordes has characters that are the 'good guys' of their faction and the 'bad guys if their faction.
Then we can have no major events with everything being border scuffles, except Cryx who keep losing Warcasters, but don't because everyone just gets resurrected making any of them dying a pretty irrelevant story point. Fluff would devolve into Every Race Versus Nids if you did this to 40K. Menoth, Khador and Cygnar suddenly fighting together is a bit like Abaddon shaking hands with Stracken so they can go fight the Necrons together.