I believe Michael having his brother Fredo killed was unforgivable and unnecessary, especially considering Michael was able to keep Fredo under watch or similar until Mama Corleone's natural death
Michael could have continued the same arrangements

The ever astute Roth "would have always remained a threat" Kill or be killed

I don't believe Frankie had to commit suicide to save his brother Vincenzo as Vincenzo was never in danger

I believe Frankie was jolted and reminded of omertà, by his brother Vincenzo's presence, his piercing icy stare was knifelike
Made Frankie realise, what he was about to do - betray his Don - is a no-no in the business they have chosen and need to honour the omertà which was exactly why Michael brought Vincenzo over

Frankie "sliced his wrists on his own accord" with 'helpful push' from Tom!