Administration needs grow with the number of believers; Stephen is arrested, gives the High Council a history lesson before telling them they killed the Messiah, so they stone him, with Saul’s passive approval: Acts 6–8:1
Telling the religious rulers that they’d not only missed God, but killed His son, wasn’t popular. But add to that a vision of Jesus in heaven:
Acts 7:55-56 But Stephen, full of the Holy Spirit, gazed steadily into heaven and saw the glory of God, and he saw Jesus standing in the place of honor at God’s right hand. And he told them, ‘Look, I see the heavens opened and the Son of Man standing in the place of honor at God’s right hand!’
And the rulers became even angrier. This was the same claim that caused them to seek Jesus’ death only months earlier.
So they stoned Stephen to death.