If Rocco was merely a Clemenza guy then why does Michael promote him? And keep him as a capo for many years??

The question is simple. If Rocco didn't kill the shooters, who did? Fredo doesn't have the ability nor the gumption to carry out two flawless murders...while restraining his hysterical wife

Sonny - Well then, business will have to suffer, all right? And listen, do me a favor, Tom. No more advice on how to patch things up, just help me win, please