Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Num 25:1-5) The Israelites enticed by the daughters of Moab and Midian.
(Num 25:6-15) Phinehas puts Zimri and Cozbi to death.
(Num 25:16-18) The Midianites to be punished.
The friendship of the wicked is more dangerous than their enmity; for none can prevail against God's people if they are not overcome by their inbred lusts; nor can any enchantment hurt them, but the enticements of worldly interests and pleasures. Here is the sin of Israel, to which they are enticed by the daughters of Moab and Midian. Those are our worst enemies who draw us to sin, for that is the greatest mischief any man can do us. Israel's sin did that which all Balaam's enchantments could not do; it set God against them. Diseases are the fruits of God's anger, and the just punishments of prevailing sins; one infection follows the other. Ringleaders in sin ought to be made examples of justice.
Phinehas, in the courage of zeal and faith, executed vengeance on Zimri and Cozbi. This act can never be an example for private revenge, or religious persecution, or for irregular public vengeance.
We read not that any Midianites died of the plague; God punished them with the sword of an enemy, not with the rod of a father. We must set ourselves against whatever is an occasion of sin to us, Mat 5:29, Mat 5:30. Whatever draws us to sin, should be a vexation to us, as a thorn in the flesh. And none will be more surely and severely punished than those who, after Satan's example, and with his subtlety, tempt others to sin.