It was going along pretty good until it turned into this retarded wild west shootout.
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I disagree. Leo wanted to control Frank and work Frank to the point that he burned himself out. Frank wanted to retain his status as an independent thief, so he had no choice but to kill Leo in order to keep his autonomy.
You've seen Guy Standeven in something because the man was in everything.

Leo deserved every gram of lead forced into his body.
Thumbs Up, Thumbs Down and a Wagging Finger of Shame

damm now I feel like a hamburger.

Because Leo was going to kill Frank's Wife, and we'd be eating her in our "Wimpy Burger" which I happen to love!

they could have worked it out
Crime bosses (in real life and in the movies) are not famous for "working things out." They are famous for "popping a cap in someone's ass." And when Leo snarls,
"why don't you join a labor union"
to Frank, that means negotiation is over.

Leo lied to Frankremember Frank was supposed to be paid all in cash and Leo tried to get him to accept a little cash and the rest in stock or whatever?
Leo sealed his fate when he told Frank how it was really going to be going forward.
Frank was not going to be Leo's punk.

True but then the ending would have been boring and wouldn't have worked if they just "worked it out".
But I agree Leo should have given Frank a lot more, he scammed Frank.

You kill my close friend and then threaten to whore out my wife and then kill my wife and kid and feed them to the public in a Wimpy Burger and you think there is room for reason?
I'm a very reasonable guy buy that wouldn't fly in my world. Whether it was Leo or it was Frank, one of them was going to die.

O.P., your take on the ending is ridiculous. Leo didn't deserve to live, and Frank killing him was a great gift to the world. Leo was a sadistic, ruthless, bloodthirsty gangster. If you felt wronged by his demise, that speaks volumes about your personality. Leo much?

What Frank (James Caan) did to Leo fits perfectly with his character. Frank is a guy who has had it with the system and the society, and had long before established his MO while serving in prison: he wouldn't put up with anyone trying to pin him down. The movie was very explicit about this, and no way would he ever chose reconciliation with the likes of Leo.
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Leo ruined Frank's dream. Frank was a talented theif and Leo took advantage of him. Frank said it as well as anyone could it was his sweat, his risk, not Leo's. I was happy when Leo got popped.

If they're going to kill you and not let you out of the game it was definitely warranted.

Leo had no intention of letting Frank go. Ever. Frank saw this and knew he had to get out.
Drifting through lost latitudes with no compass and no chart