Again, that was the old school loyalty in him. Someone had attempted to kill his Don, the Don of the Corleone family that he was a part of for so many years. So he wanted to now protect his Don, take revenge on behalf of his Don. Remember, in his mind the Corelones were like The Roman Empire.

But there is a fine line between love and hate.

Frankie was loyal to The Corleones / Michael. Michael basically brushed him off, didn't show him the proper respect under old school rules and kept him waiting in a lobby.

Michael tells him to make peace with the Rosatos without really going into detail. He basically tells him that he wants to deal with Hyman Roth, another guy that Frankie didn't trust, and then sends Frankie on his way.

Frankie had to feel slighted. Had to have some kind of suspicions at this point. But he was old school, so he stayed loyal.

Then he tells Frankie to go meet with the Rosatos and make peace. He tells Frankie that an attempt was made on his life BUT he doesn't want to retaliate. This was not old school Cosa Nostra as far as Frankie was concerned. But he followed his Don's orders anyway because he was old school loyal.

Frankie really must have some kind of doubts now. Really must have felt that Michael was up to something with Roth. But he stays loyal and follows his Don's orders anyway and goes to make peace with the Rosatos.

And what Happens. And attempt is made on Frankies life. And during that attempt he's given a message that Michael Corleone was behind this attempt being made on his life! And he's left to die. Picked up by the authorities and charged with all kinds of crimes. In his mind he stayed loyal all along and still his Don betrayed him.

So after all of these little things that happened between this old school mafioso and his new world boss, and then being told that it was his boss, who sent him out on this mission of peace, who now set him up, it's not inconceivable to turn.

When you really think about it, after all that happened with he and Michael, after being left for dead by the person that you were there for in the first place, and now being charged with all kinds of crimes, it was really the smart move on Frankie's part to turn witness. What other choice did Michael really leave him?