Hagen knew what Sonny would say. Sonny was chafing at being under the Don's thumb. He wanted a big operation of his own. Something like this would be perfect.
My view is that if Tom knew Sonny was "chafing at being under the Don's thumb," then others would know it, too--Sonny being an unsubtle fellow. That's why, if he went for the deal if Vito died, he'd fall under suspicion--not only in the Family but outside, too.
I acknowledge this is from the novel...Then, surely Vito would have known it too yet Vito continued to keep Sonny “under the Don's thumbâ€
Vito was instrumental, contributed, caused the misery of every one of his children, to their gloom and doom indeed
