John Calvin Commentary Psalms 106:38

"And shed innocent blood, Even the blood of their sons and of their daughters, Whom they sacrificed unto the idols of Canaan; And the land was polluted with blood." — Psalms 106:38 (ASV)

And they shed He denounces with still greater indignation that religious frenzy which led them to sacrifice their own children, and thus to pollute the land by the shedding of innocent blood. Should anyone object that Abraham is praised because he did not hold back his only son, the answer is plain: He did it in obedience to God’s command, so that every trace of inhumanity was erased by means of the purity of faith.

For if obedience is better than sacrifice (1 Samuel 15:22), it is the best rule both for morality and religion. It is an awful manifestation of God’s avenging wrath when superstitious pagans, left to their own devices, become hardened in deeds of horrible cruelty. As often as the martyrs risk their lives in defense of the truth, the incense of such a sacrifice is pleasing to God.

But when the two Romans, named Decii, in a detestable manner sacrificed themselves, that was an act of heinous impiety. It is not without just cause, therefore, that the prophet emphasizes the guilt of the people by this consideration: that to the corrupt way of worshipping God, they had added excessive cruelty.

Nor is there less reason for charging them with having polluted that land from which God had commanded them to expel the ancient inhabitants, so that he might make it the special place where he was to be worshipped. The Israelites then were doubly wicked, who, by not only defiling the land with their idolatry but also by cruelly slaughtering their children, robbed God of what was owed to him and, in a way, thwarted his plans.