He was an old timer, a street guy.

Let's face it, the FIRST time that we watched GFII, many of us could not understand why Michael wanted him to pretend that he is friendly to the Rosato Brothers!

And keep in mind that Frankie was probably a loyal soldier to Clemenza, and being the old school italian that he was probably loved Clemenza. So his feelings and loyalty to Clemenza got in the way and he wanted to avenge Clemenza's death, heart attack or not, by getting back at the Rosato brothers.

Michael was not the most trusting person in the world. He was not one to share his plans with those close to him, except for Neri. Michael could have reached out to Pentangeli before the kid's communion, and told Pentangeli that something was in the works that would benefit the Corleone family as a whole ("you're family is still called Corleone") and maybe Pentangeli would have kept a much lower profile.

The more I think about it the more I realize that Michael, being the egotistical and untrusting cold hearted bastard that he was, caused those around him to crack and crumble because of his lack of diplomacy to his own people. While he may have been the saviour of the family in GFI, he certainly became the destroyer of it by the end of GFII.

I don't think that it was so much that Pentangeli was dumb, just think that he was kept a little too much in the dark.