Saul hated Christians, prior to this happening he had written letters to the high priests in Damascus about any Christians around that he could lock up and take as Prisoners to Jerusalem. And yet it was him that God spoke about to Ananias as His 'chosen instrument' (Acts 9.15) to speak to the Jews and Gentiles about the gospel.
Saul would not have been my first choice, but God has an ability to see unlocked potential within us that even we can't see.
Would you, having seen a man making threats against your friends and family, befriend him? That's exactly what God did here.
And if you think about it, there isn't really any other logical explanation for why Saul would suddenly change his name to Paul and start preaching the same news that Stephen had been preaching just days previously.
What sane person would turn around from persecuting a group of people and then decide to join that persecuted group? It doesn't seem like something an ordinary person would do.
There had to be a reason for it. And there was. Saul had an encounter with God that changed his life. It wasn't an encounter that happened 'in his head' either. The men who were with him heard the voice of God too. They didn't see anything, but they heard it and they were speechless (Acts 9.7).
From this point onwards Saul was a different person. It was less of a concious thing, not so much trying to be different, but being different because what he had witnessed had affected him. It had dramatically changed his perspective.
"At once he began to preach in the synagogues that Jesus is the son of God. All those who heard him were astonished and asked, 'Isn't he the man that raised havoc in Jerusalem among those who call on this name? And hasn't he come here to take them as prisoners to the chief priests?'"
The great thing was that Jesus wasn't concerned with the man that raised havoc in Jerusalem with Christians, Jesus was concerned with the man preaching in the synagogues about Himself.
Where we come from ceases to matter in the light of who God is. We can say that we've sinned. We can say that we've blasphemed. We can even say that we've killed, or endorsed it, as Saul had. But in the light of God's mercy, it doesn't matter who we were before.
Meet Jesus and have a Saul to Paul conversion. God could do great things with you if you'd just let Him.