Grama at the end

On the surface I see where you’re coming from but if you peel back the layers the scene was not out of place.
Grama was at the very least a sociopathic and merciless loan shark, pimp and someone who violently abuses animals without a second thought, getting paid wasn’t enough for Grama, he wanted Mike to lose everything, to include being seriously hurt and/or killed.
He wanted his cake and to eat it too.

Grama had been played this is why he was annoyed.

His line of work is about reputation and what went down will hit the streets also I expect KGB wont be happy so who will he take it out on Grama.

Hey, funny enough the thread beneath yours is asking the same thing, you may find some more/different insights there.
https://moviechat.org/tt0128442/Rounders/58c7779493cef4080d7c6ac0/Why-is-Gramma-pissed-spoilers

I'm thinking it was just for dramatic effect and not necessarily thought through.

There's the good guy and the bad guys. The good guy won, the bad guys get pissed.

It doesn't make sense given the context, but it made for a dramatic scene.

The facts of the matter are that Grama bought Worm's debt for pennies on the dollar. KGB backed him. Mike now has enough money to pay Grama the full amount of the debt, minus what KGB staked, which should be a significant profit.

But bad guys get pissed emphasizes good guys won.

A 9MM would've taken care of Grama, but I enjoy this movie alot.. A very young Matt Damon indeed

Grama's over-the-top reaction in Rounders does seem intense, but it makes sense when you break it down:

Grama's Ego and Investment: Grama's anger isn't just about the money—it’s about pride and control. He operates in the shady world of gambling and loan-sharking, where losing face is bad for business. Mike beating KGB represents a loss of power for Grama because it undermines his ability to intimidate and control people in his orbit.

Worm’s Influence on Mike: Grama despises Worm, but Mike represents a lingering connection to him. Mike’s victory cuts Grama’s influence over Mike, essentially severing Grama's ability to collect or manipulate Mike further. It's like watching the last piece of leverage he had slip away.

Grama's Reputation: The scene implies Grama’s anger might also be about perception. He bankrolled KGB's initial winnings against Mike and expected to profit from it. Mike walking away scot-free signals a failure on Grama’s part, which could tarnish his reputation as a loan shark and enforcer.

Unpredictable Temperament: Grama is shown to be a volatile character who acts out physically when things don’t go his way. The table smash fits his personality—someone who thrives on fear and violence to assert dominance.

It’s less about him getting what he wanted and more about losing control of the situation. Mike’s win and subsequent departure likely feel like a public slap in the face, and Grama can't take it.