I think the Roth character sums up much of the theme from GFII: treachery & deception. I agree with the post stating that part of it was revenge for the killing of his friend Moe Green. But perhaps just as important was that Roth could see how Michael was taking over several casinos in Vegas. Roth was certainly smart enough to realize that he could never survive a fight with a mafia family with the depth of the Corleones. If you can't beat 'em, join 'em. By creating that appearance, Roth was able to get close enough to Michael in an effort to have Michael let his guard down. When that happened, Roth thought that he could make a second attempt on Michael's life. So it made good business sense for Roth to join Michael. It also helped keep Roth protected.

If you've seen "Casino," the example of DeNiro's character, Sam Rothstein, can be viewed as a parallel to Hyman Roth. Both Rothstein & Roth were Jewish. They didn't want their profitable businesses shaken down by the mob. Instead of fighting the mob, they chose to become partners. I'm paraphrasing here, but during the narration of "Casino," Pesci's character said that keeping the mob boss happy with money was the best insurance policy that Sam Rothstein could ever buy.