Consider that Saul of Tarsus may not have been a very zealous Pharisee. That might explain the conundrum of why a Sadducee high priest would hire a Pharisee, and why a Pharisee would work for a Sadducee.
The movement that the high priest hired Saul to put down, was a "coming kingdom" movement, which was a Pharisee doctrine, not shared by the Sadducees.
How else could Pharisee Saul go about killing or imprisoning people who were zealous for the very teachings of true Pharisaism? Remember that Jesus endorsed the teachings of the Pharisees, and only criticized that generation of Pharisees for their hypocrisy.
During Saul's association with those he was killing or imprisoning, he realized that their goal of "the coming kingdom", was the same as the goal of true Judaism(before it became backslidden). Saul had a revival of his "true Jewish" beliefs, and simply returned to the "true Judaism" from which he, and Jerusalem Jews, were backslidden.
The Jerusalem apostles were simply fishermen who had been given a crash course in how to preach the gospel which God preached first to Abraham, but Saul already understood that gospel to the core. The Jerusalem apostles preached from memory, but Paul preached from understanding. Paul's version of the gospel was bound to be much deeper than the very basic gospel teachings of the other apostles.
This may be why there was a continuing conflict between Paul and the Jerusalem apostles, making Paul ready to disown the Jerusalem apostles, but not making the Jerusalem apostles ready to disown Paul.
The gospel that Paul preached was probably more concise, and therefore less prone to false interpretation, as the gospel of the Jerusaiem apostles was.
It is clear from Paul's writings that he was making "true Jews" out of gentiles, and backslidden Jews. Their hearts were being circumcized (circumcision meaning Abrahamic covenant acceptance), and that these true Jews were Abraham's children by virtue of hearing a believing the gospel which he heard and believed first.
The first we hear the gospel is when God revealed it to Abraham: A great nation of Abraham's children, will inherit all the land between the Euphrates and the river of Egypt for an everlasting possession, and will bless all the nations with (peace on earth, good will to men).
It is quite obvious that Rome would have to suppress that gospel, because the land promised in the Abrahamic gospel, was currently part of the Roman empire. Saul had been hired by the high priest, who served at the pleasure of Rome, to put down the Abrahaamic gospel/coming kingdom movement.
The territorially defined chosen nation which fell 1000 years earlier, obviously had not achieved "everlasting possession" of the promised territory, so now a new body must be assembled, to follow the coming "son of David" to resurrection of the "kingdom of covenant Israel" so that he can be "given the kingdom of his father David", and can fulfill the Abrahamic gospel by bringing "peace on earth, good will to men".
All traces of the true gospel were wiped out at the first ecumenical council of Nicea.
What do you think?
"The truth will set you free, but first it will piss you off!!!"