Pharisees 
The context in Sotah a list of false saintly actions and then "Beware of the Tzvuin (the fakers) who do the actions of Zimri and then request reward like Phineas" does imply that there were a class of false pietists.   

What was the Mishna in Avot you mentioned? (RE: Paul\Saul there is some form of tradition that he was actually a Pharisee who was sent by the Rabbis to seperate the Christians from the rest of the Jews (because the early Christians were very similar to the Jews). They say for instance that he initiated the change from Saturday to Sunday (I thought Constantine did that.)