TB, Fredo didn't have to die. Michael had effectively expelled him from the family and the family business. He was no threat. Michael's murder of him was simply revenge which in III he came to regret. However, Fredo's murder repeats a pattern: Michael murdered Carlo although Carlo could have been expelled from the family and eliminated as a threat. As in the case of Fredo, Carlo's murder was simply revenge.

It's probably true that Roth had to die. There was no way for Michael to isolate Roth. Thus, Roth would have always remained a threat.

Frankie's death also was revenge, born out of necessity - by Frankie. Frankie had to commit suicide to save his brother.

In the end, as we've opined many times, the world in which those people operate is so, so different from ours. Their values are so, so different from ours.