Listen, as much as Zeb was wise and honorable in Honorable ones, I think that viewing him as the sane and collected one out of Kallus and him is a huge misrepresentation of his character. Don't get me wrong, I love me some level headed, calm, honorable Zeb. BUT
The guy was in his late thirties, beefing with a 14 year old and LOSING!! And it kept going as they got older! Not only that, he managed to have deeply mutual beef with Chopper despite understanding ZERO binary. He simply assessed that Chopper's tone was bitchy enough to warrant beef! If someone annoys him enough, he will be ready to fight them on it! Does not matter who it is.
Like he is level headed, calm and honorable when he's dealing with levelheaded, calm and honorable people - most of the time. But he's also constantly ready to switch that out for angry, bitchy and petty at the turn of a dime, if provoked! Which wouldn't be a problem if it wasn't for the fact that he... Has a bit of a short fuse. Like the guy does have anger issues.
Idk I definitely don't feel like Kallus would purposely provoke him much, but unintentionally? I could totally see Kallus stumbling and managing to push all of Zeb's metaphorical buttons. They just sorta clash in some ways, especially at first. And Kallus is on his best behavior, or at least trying to be - but isn't that fucking annoying?! You're trying to beef with a guy and he can't even give you the time of day for that?! Zeb was peeved and now he's pissed!
Idk I just really want more stuff with Zeb and his anger issues, idk I think it adds a nice flavor to their dynamic. Like yes he took pity on Kallus on Bahryn, but also Kallus laughed at him for falling on his ass and Zeb hit him with the raw ass line "how would you like some more broken bones?"