Accuse Pentangeli of what? Nothing. Instead, he tells him with an irate demeanor that his attempted assassination took place in his home, etc. Since Pentangeli is in charge of the Corleone family activities in NY, why wouldn't he utilize Pentangeli to take his revenge?
I believe that he may have meant to use the word "imply" instead of the word "accuse" in his question.
And he actually did utilize Frankie to take his revenge...only he did it in his own manipulating Michael way!
It wasn't really an irate demeanor. It was an act. In Michael's rant he purposely implies to Frankie that he suspects him without actually coming out and accusing him. Michael obviously puts on that LOUD RANT to put the thought in Frankie's head, even just for a moment, that he suspected him and was possibly facing the wrath of his Don. Michael did this knowing that in his fear Frankie would now agree to do whatever Michael asks of him. And how does Frankie react? He plays right into Micahel's hands :
PENTANGELI : "
Now -- Jesus Christ, Mike, Jesus Christ. Look, let's get 'em all -- let's get 'em all now, while we got the muscle."At that moment Michael speaks softly, getting all nostalgic with Frankie. At that moment, by Frankie's reaction and his reply, Michael knows that he can now get Frankie to go and make the peace with the Rosato brothers.
All Michael wanted was for Frankie to agree to go to the Rosato Brothers to make the peace. And in the way that Michael intentionally goes off in that scene, he manipulates Frankie into agreeing to go to the Rosato brothers, which translates into utilizing Frankie to help him get his revenge on Roth.
