Simply because this young man called Saul had a future ahead of him. Today you and I know him not as Saul but as Paul, Saint Paul, the influential and energetic apostle to the Gentiles. But this proponent of faith in Christ started out as an angry young man. Young Paul's religious anger didn't go away after the execution of St. Stephen. He went to the chief priest and asked him to write letters of authorization to the synagogue leaders in the city of Damascus.
Psychologists say that anger wears many masks.
You keep it bottled up and that ulcer or headache might be the result. That's one example of a way some people mask their anger. There are other ways people disguise or hide their anger. One of those is the religious mask or you may call it spiritual or you may call it moral. The morally angry person thinks, 'I am mad at you.