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.

• Rocco

Did Rocco have to die? What would our take be as to why Rocco was killed off? It didn't morph into anything

Rocco was the only one of Michael's men who didn't return -- died on a successful job
[I am in the minority! that killing Roth need not have been a suicide mission at all. Rocco had no escape plan whatsoever]

He need not have died

Roth? Yes I can understand that. Pentangeli? I don’t think so. He was no threat to Mike anymore after recanting and knowing what would happen to his family if he tried again. Fredo? Absolutely not. There is zero evidence that shows Fredo knew about the plot to kill Mike, there is plenty of evidence to show he was misled and used. Was he stupid? Of course but was it worth killing your own brother? What did it accomplish other than causing more pain for everyone including Mike eventually

Don't know why Rocco was killed off? True didn't morph into anything

I too reckon killing Roth need not have been a suicide mission at all.
He was like a deer caught in headlights, having no escape plans whatsoever

Drama trumps logic every time.