Frank Pentangeli was loyal in The Godfather: Part 2. He obeyed Michael's order and went to make peace with the Rosato brothers. But then there was an attempt to kill Frank and it looked like Michael ordered it to Frank. So this incident might influence Frankie to betray Michael. But in depth he was loyal. He even backed off from his witness thing.
Still Michael ordered and killed him. Didn't he understand that Frankie Five Angels was loyal? Why didn't he feel any sympathy for Frankie? What was the motive? What am I missing?