There are strong grounds for believing that, if Paul was not a member of the Sanhedrin at the time of Stephen's death, he was elected into that powerful senate soon after as a reward for the zeal he had shown against the heretic.
A zeal without knowledge
Paul himself says that in Jerusalem he not only exercised the power of imprisonment by commission from the High Priests, but also, when the Christians were put to death, gave his vote against them. From this expression it is natural to infer that he was a member of that supreme court of judicature. Paul's zeal in conducting the persecution was unbounded.
We cannot help observing how frequently strong expressions concerning Paul's share in the injustice and cruelty now perpetrated are multiplied in the Scriptures. In Luke's narrative, in Apostle Paul's own speeches, in his earlier and later epistles, the subject of his persecution arises again and again. He "made havoc of the Church," invading the sanctuaries of domestic life and "entering into every house" (Acts 8:3, 9:2). Those whom he thus tore from their homes he committed to prison with the express purpose that they would die.
For this very reason, I (Paul) truly thought in myself that I ought to do many things contrary to the name of Jesus the Nazarean, Which I also did in Jerusalem; and many of the saints I shut up in prisons, having received authority from the chief priests; and when they were put to death, I gave my full consent against them (Acts 26:9 - 10, HBFV).
And not only did men thus suffer at Paul's hands, but women also, a fact three times repeated as a great aggravation of his cruelty (Acts 8:3, 9:2, 22:4). These persecuted people were scourged "often" in many synagogues (Acts 26:10).
Stephen was not the only one who suffered death, as we may infer from the Apostle Paul's own confession. And, what was worse than scourging or than death itself, he used every effort to make them blaspheme that Holy Name whereby they were called.
Paul's fame as an inquisitor was notorious far and wide. Even at Damascus Ananias had heard (Acts 9:13) "how much evil he had done to Christ's saints at Jerusalem." He was known there (Acts 9:21) as "he that destroyed them which call on this Name in Jerusalem."
It was not without reason that, in the deep repentance of his later years, Paul remembered how he had "persecuted the Church of God and wasted it" (Galatians 1:13, Philippians 3:6) and how he had been "a blasphemer, a persecutor, and injurious" (1Timothy 1:13). From such cruelty, and such efforts to make them deny that Name which they honored above all names, the disciples naturally fled.
In consequence of Paul's persecution against the Church at Jerusalem, they were all scattered abroad throughout the regions of Judea and Samaria. The Apostles only remained (Acts 8:1). But this dispersion led to great results.
The moment of lowest depression, brought in part by Paul, was the very time of the Church's first missionary triumph, "They that were scattered abroad went everywhere preaching the Word" (Acts 8:4, 11:19 - 21). First the Samaritans, and then the Gentiles, received that Gospel, which the Jews attempted to destroy. Thus did the providence of God begin to accomplish, by unconscious instruments, the prophecy and command which had been given in Acts 1:8.