Mike the Executioner
What went on up there? Poppers and weird sex!
This is one of the dumbest decisions Tony makes in the entire series.
He beats up Zellman for dating a woman he doesn't care about and hasn't in a long time, because he's drunk and that Chi-Lites song was hitting him just right. It ruins Zellman's relationship with Irina, who thinks that Tony is still in love with her and meanwhile, he ends up sleeping with her cousin. Irina finds out and tells Carmela everything, which causes the separation.
One of Tony's biggest flaws is hating the idea of seeing other people happy, and in this case, his inability to move on almost destroys the family.