Prior to becoming a Christian, Saul of Tarsus hated Christians. He made it his goal to capture, then bring Christians to public trial and execution.
1 And Saul was consenting unto his death. And at that time there was a great persecution against the church which was at Jerusalem; and they were all scattered abroad throughout the regions of Judaea and Samaria, except the apostles.
2 And devout men carried Stephen to his burial, and made great lamentation over him.
3 As for Saul, he made havock of the church, entering into every house, and haling men and women committed them to prison.
(Acts 8:1-3)
Saul believed that he was serving God by finding Christians, throwing them into prison, and even executing them when possible.