How was Paul able to imprison and kill Jewish believers before his conversion?
Paul imprisoned believers by authority received from the chief priests, and he voted to put them to death. Rome permitted Judaism to exercise authority over its own people, so Jewish authorities could treat the new believers as heretics and execute them; Rome would not have allowed such treatment of a Gentile.
Les teaches that Paul took responsibility for shutting up many of the saints in prison, having received authority from the chief priests, and when they were put to death, he agreed. Those Jewish believers were killed as heretics, viewed as the offscouring of Judaism. Les explains that Rome respected ancient religions, including Judaism, and even guaranteed safe delivery of Jewish offerings sent to the temple. Because Rome condescended to Judaism and gave permission for Jews to put their own people to death, Paul could imprison and vote to execute Jewish believers. Rome would never have allowed that for a Gentile. Later, Christianity appeared as something new, not an ancient religion, so Rome tried to stamp it out, bringing Christians under massive persecution while the Jews did not face the same.