Divide and Conquer: Caesar advises Crassus to do this, essentially laying the seeds for Gabrielle mistrusting Xena. It doesn't work.
I Owe You My Life: Vercinix wanted to kill Crassus in the cell, but he was willing to back off for now as Gabrielle had just saved him.
It's Personal: Vercinix hates Caesar for attacking his home, but he hates Crassus even more for the slaughter of Gardenis (where Vercinix's brother and that man's family were killed).
My God, What Have I Done?: Gabrielle willingly lets Crassus get executed after learning the truth about Gardenis. She's disgusted with herself afterwards. In turn, Xena regrets putting her in that position.
Caesar is looking out for his own power base both when trying to save Crassus and then leaving him to die.
Pompey makes his own appeal to Xena—saying their mutual dislike of Caesar could make them allies.
The Reveal: Crassus had claimed that he had nothing to do with the slaughter of Gardenis. Vercinix reveals that Crassus personally ordered it—being told that by a crucified niece before she died.
The Rival: The Triumvirate is described in these terms.
Xena: Caesar, Pompey, and Crassus share control of Rome and its provinces. Each serves to balance the other. They hate each other, but they can't live without each other.
Sadistic Choice: During the climax, Caesar has to decide if Crassus (thought by the public to be Vercinix) gets executed. If he does, then he loses his one big check against Pompey. If he doesn't, he loses face in front of the people and taints his recent victory in Gaul. Ultimately, he lets Crassus die.
Xanatos Speed Chess: With Crassus rumored to be dead, Pompey was chomping at the bit to grab more power, including even trying to ally with Xena. During the execution, he can see that that's Crassus in place of Vercinix. He doesn't interfere, though, and even mockingly asks Caesar what he's going to do now. Whether or not Caesar reveals the truth, Pompey benefits: either one hated rival is effortlessly killed and frees him up to grab more power, or another hated rival is publicly embarrassed during a celebration of Caesar's victory in Gaul.
You Have Failed Me: Caesar had earlier mocked Crassus for being captured by the woman he himself captured years ago. He still tries to save him despite this (as he really needs the balance against Pompey), but when the situation threatens to make him look foolish to the public, Caesar reluctantly lets Crassus die.