“[For Paul]... that Jesus of Nazareth could be the expected Messiah, as his disciples maintained, was out of the question. The conclusive argument (in Paul's mind) was simply this: this Jesus had been crucified. A crucified Messiah was a contradiction in terms. Whether his death by crucifixion was deserved or else resulted from a miscarriage of justice was beside the point. The point that he was crucified clearly came within the meaning of the pronouncement of Deuteronomy 21:23 "a hanged man is accursed by God." In fact, a crucified Messiah was worse than a contradiction in terms; the very idea was an outrageous blasphemy.
The Torah, the law, the customs, the ancestral traditions, and everything that was of eternal value in Judaism, was imperiled by these disciples activity, claims, and teaching. So, here was a malignant, cancerous growth which called for drastic surgery. The defense of all that made life worth living for Paul was a cause which engaged all the zeal and energy of which he was capable. When the chief priests and their associates launched their attack on the disciples of Jesus, Paul came forward as their eager lieutenant. Their motives may have been partly political and self-seeking, his were entirely religious. Nevertheless, their actions provided him with the occasion to protect the interests of the law he loved.”
--F.F. Bruce
The Apostle of the Heart Set Free