It is a great question. Why did Paul put so much effort into persecuting his fellow Jews that made up the early church of Christian believers?
Studies of recent years have shed much light on the issue of Jewish diversity on the Second Temple context - and this diversity was tolerated by others. (Well, some did so differently - e.g. the exclusive nature of the sectarians at Qumran).
Dare I postulate that, to borrow the words of Pliny, they revered Jesus and sung "to Christ as if to a god."
What else would have got a young Pharisees knickers in such a knot?