I feel Fredo DID have to die. Why? This was the second time he had taken sides against the family (the first was with Moe Green in Part 1). He proved he can't be trusted. His outburst about "being passed over" showed that he resented Michael. Given how easily he was hoodwinked by Roth, letting him live could have proved very dangerous as he might have easily been tricked again later on by another smooth-talking gangster. He could have gotten enough resentment that he would try to sabotage Michael's endeavors, even try to show that Michael was a criminal. The possibilities are endless for what he might have done had he been left alive.

Roth absolutely had to be killed. He was far from a harmless old man like he pretended to be and would have been a major thorn in Michael's side one way or another if he had been allowed to live.