The Corleones certainly set Geary up by drugging him in the whorehouse that Fredo operated, then having Neri kill the prostitute (notice him washing his hands, like Lady Macbeth--"out, out, damned, spot--out I say"
), then having Geary wake up with the dead, bloody hooker in his bed--and no memory of how it happened.
Geary had a minor role in the Michael/Roth fandango. Roth, pretending to be Michael's kindly surrogate father, has Johnny Ola tell Michael that if he wants take over Meyer Klingman's piece of the Tropicala hotel that Roth secretly owns with the Lakeville Road Boys, Roth won't object. Since Roth planned to have Michael killed that very evening, he was, in effect, giving away the sleeves of his vest. Both Roth and Michael knew that Geary was the most politically influential man in Nevada, and that Michael would need Geary's permission to "get on the license" as Klingman's successor. Michael contacted Turnbull, who was undoubtedly Geary's political bagman, to arrange Geary's permission. But Geary held out for $250k plus 5% of the gross of all of Michael's hotels.
Now a nice question presents itself: Did Geary try to hold up Michael for all that money just because he was greedy and disliked Italians? Or did Roth influence him to put the bite on Michael? I'm guessing it was a combination of the two. Geary certainly was greedy and prejudiced. But I also think that he must have been tight with Roth, since Roth was already established in Nevada and had undoubtedly done plenty of satisfactory business with Geary and Turnbull. Roth, clever fellow, probably played on Geary's greed and prejudice by telling Geary: "Hey, look, this Corleone guy is a bigshot Mafioso; he's loaded with money and he can pay you a lot more than he negotiated with Turnbull. You oughta get more, too, since you're being so nice by giving a dirty outfit like the Mafia a foothold in this 'clean country'." Meanwhile, Roth was manipulating Geary to put a stumbling block in Michael's plan to acquire an interest in the Tropicala--so that even if Michael had been assassinated, his successors still wouldn't have an interest in Roth's hotel.