My take, for what it is worth!

Vito was slipping!

among others, complacent, out of touch, underestimating his main rival Barzini, the puppet master, who was playing the Corleones like a fiddle, hiding behind Tattaglia [who was later dismissed as a pimp] until the peace meeting

Vito was instrumental, contributed and caused -

Vito knew Santino was a bad Don yet if Santino had not been killed he would have still become the Don

Vito knew Tom was not a wartime consiglieri yet Vito made Tom, good lawyer, not a Sicilian, the consiglieri albeit in 'peace' time

Vito's dismissal of Fredo “And Fredo, well, Fredo was, well...” yet Vito did not pay attention or nurture Fredo to at least make Fredo feel he was playing an important role in the family business

This failure of Vito, made Fredo resent, jealous etc. towards Michael, Fredo's kid brother, so much so that Fredo turned traitor and arguably conspired Michael's murder to take over the Donship

Vito had such contempt for Carlo yet he allowed Carlo and Connie's marriage to go ahead
Vito giving Carlo just a living “Never [give Carlo something important] Give him a living but never discuss the Family business with him”

This failure of Vito, made the low life parasite, wife beater, resent so much so that Carlo was beating Connie even when she was pregnant and setting Santino up to be murdered albeit revenge for Santino's public beating and humiliation of Carlo which may not have happened if Carlo had been given something important