Moreover, and perhaps most significant, we see the people reject the leader God has given them. God chose Moses, empowered him, worked signs and wonders through him and delivered them through him. Now, they turn their backs on him, demanding that their own desires be met. It becomes clear that there are grave consequences for rejecting God's chosen man.
This is not meant to be prescriptive to God's people in all cases. Killing is a violation of God's commandments. Our lesson is not about killing but about the ruthless removal of anything unholy from among God's people. In this incident we see that sin will not be tolerated within the camp. There is a grave and serious price to pay for rejecting God.
We are at a watershed moment in the story of the Exodus. God has delivered His people and given them the Law. At the moment they are receiving the Law, they are called to make a decision. Those who decide not to follow God and the leader He has designated, are eliminated. This will become another pattern we will see throughout the rest of Scripture. The Law has done what it is designed to do, reveal sin.
In another significant turn, this is the first act of the Levites' complete devotion to God and is immediately followed by the Lord's blessing. They are ordained into His service and will eventually play a significant role in the history of Israel as priests and servers in the Tabernacle/Temple. Here's another pattern we've seen and will continue to see - obedience is followed by blessing. Radical obedience is followed by radical blessing.
God sends a plague, as well. God is compassionate and merciful but will not tolerate open, unrepentant sin. There will always be earthly consequences for sin among God's people. They remain His people, but becasue of their willful self-indulgence, there will be some suffering.