From First and Second Timothy, Titus by George T. Montague, SM, commenting on 1 Timothy 1:12–17:
Paul had been a blasphemer. How can he say that, if indeed he had thought he was acting in God’s behalf in rounding up and imprisoning Christians and even approving the stoning of Stephen (Acts 8:1)? In retrospect, and in the light of his conversion, he now sees how mistaken and blind he was. The Pharisees believed in the resurrection of the just but thought it would happen only at the end of the age, whereas Christians were proclaiming that it had already begun in Jesus. The Holy Spirit too, according to Pharisaic expectations, would be given at the end but not now; for now, the law was sufficient. But the disciples of Jesus were claiming that they—all of them—had already received the Holy Spirit.
Furthermore, they were actually worshiping Jesus as mara (“Lord”), and to Paul’s monotheism this was blasphemous. They even claimed to be eating his flesh and drinking his blood! So from a human point of view it is understandable that he had persecuted the Church with “zeal” (Phil 3:6), “beyond measure and tried to destroy it” (Gal 1:13). But his conversion had turned his understanding of blasphemy on its head— it was blasphemous of him to deny that Jesus is Lord, risen from the dead and giver of the Holy Spirit. Even to persecute the disciples of Jesus was now blasphemy, for that meant persecuting Jesus himself—as we read in Luke’s account of Paul’s encounter with Jesus on the road to Damascus (Acts 9:5).
Arrogant has the note of excess, of extreme violence. The book of Acts describes Paul’s violence as imprisoning men and women and voting in favor of their death (Acts 8:1; 9:2; 22:4; 26:10). But he received mercy because he acted out of ignorance. The Old Testament already acknowledged the difference between sins done in ignorance and those done with evil intent (Lev 5:18; 22:14; Num 15:22–31), the former being capable of atonement. Yet the apostle is appalled at the objective evil he has done. It is not diminished but made understandable by his subjective blindness. It is characteristic of converts to see their past sins as having an enormity that they did not realize when they were committing them, even if they were excusable, or partly so, on the grounds of ignorance or malformed conscience.