While I agree that Sonny's temper led to his demise, the Corleone enemies knew that his temper was his greatest weakness and used it against him. That's where I think Tom was at fault for not being able to tell Sonny "cool it." The enemy laid the trap with Connie's beating by Carlo, hoping it would ignite Sonny into blind rage. While Sonny was this mad, he was capable of anything. Tom had an obligation the Corleone Family to rein in Sonny's anger and make Sonny think of the consequences of his actions as they applied to the Family. This did not happen - Sonny brushed Tom off after receiving the call from Connie and Sonny was ambushed at the causeway.