The "You guys lied to me" could mean anything. It's fun to speculate but there's really no indication in the film on what he means.

Maybe he said it because Ola promised him that there would be no violence. Or maybe, as TB has said in the past, it meant that it was going to be a kidnapping, not a hit. Maybe he meant what Lana said, but I don't think the Donship theory stands up to scrutiny.

Really, that would be an absurd thing for Roth to promise. How could he make that happen? Did Fredo envision Roth telling people that he killed Michael and now wants Fredo to be Don?

There seem to be varying opinions of Fredo's intelligence in this thread, but we know that Fredo is aware that "everyone says" he's dumb. Does he truly think that those same people would sit back and let him take over after Michael's death? That Rocco, Neri, Tom, etc. would stand aside for him? Even the dumbest version of Fredo couldn't have believed that, and wouldn't have trusted such a promise from Roth.

I think it was much more of a "We'll make it worth your while" hint than an explicit promise from Roth. But, if a promise was made, I think it wasn't the Donship, but more likely the promise Roth dangled in front of Michael - control of Roth's Havana empire upon his death.