I don't think Warren had motives other than retaliation for Gen. Smithers ordering the execution of the black POWs. Warren was ready to kill Smithers at the point when Smithers confessed to having ordered the execution:
Warren: You captured a whole colored command that day. But not one
colored trooper made it to a camp, did they?
Smithers: We had neither the time nor the food. Nor the
inclination to care for Northern horses, and least of all Northern
Smithers: So we shot 'em where they stood!
Warren had his hand on his gun at this point but Oswaldo Mobray immediately defused the situation. Mobray then notified Warren that it is illegal to simply kill Smithers:
Mobray: Gentlemen! Gentlemen, I know Americans aren't apt to let a little thing like an unconditional surrender get in the way of a good war. But I strongly suggest we don't restage The Battle of Baton Rouge, during a blizzard in Minnie's Haberdashery.
Mobray: Now, my Nubian friend, while I realize passions are high, that
was a while ago. And if you shoot this unarmed old man, I guarantee I
will hang you by the neck until you are dead, once we arrive in Red
Chris Mannix: I damn will guarantee that too.
John Ruth: Yeah Warren, that's the problem with old men. You can kick
'em down the stairs and say it's an accident, but ya' can't just shoot
So Warren devised a plan to kill Smithers in "self-defense", to make his killing of Smithers legal.
Narrator: Chris, John Ruth and Oswaldo, had a vigorous debate about the
legality of the self-defense murder that just transpired.
Narrator: Maj. Marquis Warren who is supremely confident about the legality
of what just transpired, ignored them, sat at a table by himself and