Interesting question.

You could make a case that Roth was the aggrieved party. Michael had been horning in on Roth's Nevada gaming empire ever since he returned from Sicily. He had Roth's best and most trusted friend, Moe Green, killed, in order to gain control of Moe's hotel. By the beginning of II, Michael owns or controls three Nevada hotels; is about to gain control of another hotel that Roth is a part owner of, and is negotiating for control of Roth's Havana holdings. It looked like Michael wanted to push Roth completely out of the gaming industry that Roth helped found.

On the other hand, Michael, who could have used violence or murder against Roth, didn't--he was negotiating for the Havana properties. Roth, on the other hand, set up the machine gun attack at Tahoe, and didn't care if Kay was killed in the attack. Moral of story: No honor among thieves.