Why are religious fanatics so cruel?
Religious fanatics become cruel because they are committed to their religion coming out on top, caring nothing for what happens to their victims. This mindset permits inhuman and torturous acts, as seen when Saul persecuted believers, forcing them to blaspheme and voting for their death, all in the name of religion.
Les explains that there is nothing more torturous or inhuman than religious fanatics, because they care nothing about what happens to their victims as long as their religion comes out on top. In the context, Paul (then Saul) admits he received authority from the chief priests, voted to have believers put to death, and punished them often in every synagogue, compelling them to blaspheme. He forced Jewish believers, likely by threat of torture, to renounce their faith in Jesus. Being exceedingly mad against them, he persecuted them to strange cities and went to Damascus to drag them bound back to Jerusalem for trial by the Sanhedrin and chief rulers, where he could demand their death. Les uses this as an example of how religious fanaticism produces cruelty.