Numbers 12:1-2 And Miriam and Aaron spake against Moses because of the Ethiopian woman whom he had married: for he had married an Ethiopian woman. And they said, Hath the Lord indeed spoken only by Moses? Hath he not spoken also by us? And the Lord heard it.
The brother and sister of Moses himself spoke against Moses, the man of God. In doing so, they rebelled against God and against the authority placed over them by God. It is a sin to speak against anyone (8th Commandment), and also to rebel against authority (4th Commandment).
Moses had married Zipporah, a Midianite, but she was called an Ethiopian (Cushitic), which simply meant a foreigner. Moses did not have much choice since he was away from his people for 80 years. But whether he did something good or bad is not the point. The point was that Miriam and Aaron spoke from a jealous spirit against the Lord’s anointed. As a result, Miriam became leprous, though eventually Moses prayed and she was healed.
When David had an opportunity to kill his bitter enemy, Saul, he refused, saying, Touch not the Lord’s Anointed. Although it might seem to the world that he had the excuse of self-defense, David would not defy God’s representative authority.
Submission and obedience to God and to God-given authority is the godly response to the grace and salvation of God. Life runs better when children, citizens, servants, employees, and laymen submit to the authorities chosen and placed by God. Jesus submitted to the will of the Father, and saved our souls.