Why so much disappointment if Clemenza turned? Tessio turned.
It was the smart move. Tessio was always smarter.
That discussion at Vito's funeral shows that the possibility of Clemenza turning HAD already crossed Michael's mind.
As well as the scene in the Corleone family den where Clemenza indirectly rebels against Michael's decisions, and Vito has to calm them down, get them back in line.
The seeds of Clemenza betraying the Corleone Family were planted in GF I.
