Sure thing Turnbull “greed for money, and lust for revenge, blind men to the consequences of their actions”
Also underestimating their opponents and overestimating! themselves

Michael was nearly undone himself with Roth

They all seemed a good idea at that time! and they all “envisioned” getting away with it

Paulie must have thought Paulie's plan of previous sickies pattern leading up to Vito's murder attempt would let him get away with it or at least been given the benefit of the doubt

Greene was under the impression the Corleones were washed up and Greene can make a deal with Barzini to keep his Hotel

Roth's clean He's never been busted for the rackets Great man Roth acts like Michael is his son, his successor
Why would Michael suspect Roth?!

Agree lucab19 some backstory [not all!] in the book enriches the story arc, our understanding of the characters' dilemmas and undoubtedly the Forum debates
Neri's backstory as to why Neri is so fiercely loyal to Michael Neri's unshakable loyalty