Paul was a murderous jew whose life's work was killing Christians until he was DIRECTLY bitch slapped off his donkey on the road to damascus and God himself asked him why he was doing something so evil.
Sometimes the most evil CAN change in an instant, but it takes God's direct intervention.
Saul became Paul (and the most prolific writer in the bible) after a lifetime of persecuting the very people he would go on to preach to / about / at.