Why did Warren treat the old General Sanford that way?

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 n*s!

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 Rock.

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 'em.

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 drank brandy.

You may recall that Sanford was a Confederate officer and Warren was a Union soldier. There was a blurb in there about how Sanford led an army that captured Warren's unit, and how Sanford left the black Union soldiers outside to die in the cold.

As such, Warren had an instant dislike of Sanford and wanted to kill him. he pushed Sanford with a story about how his son died until the only course of action was to have Sanford draw a gun and Warren could use that as an excuse to kill him in self-defense.

(NOTE: Some of the facts may be a little off, i.e. I don't remember exactly what happened to those prisoners of war, but it's close enough so the answer is still correct)

Correct me if I'm wrong, but I think that that man is not General Smithers at all. In the V chapter, Domergue's brother tells him "And when it comes to that pile of n------ we building out back, won't take nothin' to make you General of it". That's why he apostrofies he is general, and Chris Mannix simply mixes him with real general. Later, when Warren explains how he humiliated his son, Smithers simply says "You don't know my son".