In the New Testament, where would I find the Scripture that commands believers to execute heretics and witches?

There are no New Testament scriptures that commands believers to execute anyone. The Lord Jesus Himself said pay back evil with good. We need to love everyone, even those who's beliefs are totally opposite and against Jesus. It's up to us to share God's love with the world and witnessing the Gospel of love through our deeds and declaration of Scripture and allowing the Holy Spirit to convict who haven't yet met with Jesus.