“by my spirit.” This verse is saying the people acted on their own, not by revelation from God, which came through the gift of holy spirit that God put on the prophets and some priests and leaders. [For more on the gift of holy spirit, see Appendix 11, “What is the Holy Spirit,” and also commentaries on John 7:39 and Eph. 4:4. Also, the book, The Gift of Holy Spirit: The Power to be Like Christ].
“so that sin is added to sin.” The Hebrew can be translated as a result clause, “so that,” or as a purpose clause, “in order to.” Here the result clause is the most likely meaning.