But Saul, still breathing threats and murder against the disciples of the Lord...
Acts 9:1

Saul, later known as Paul, was so zealous in his hatred of what he perceived as idolatry against YHWH, that he became an idolator himself (elevating interpretations of the Law above the Law-Giver) and set *himself* against YHWH. It's good to be zealous for God's Law, but never forget that the Law was written to serve the people and their relationship with YHWH. People can be wrong and still be striving to do their best to obey and honor God. They're not pagans and they're not your enemies just because they disagree with you.

See also John 16:1-3.