John Calvin Commentary Joshua 10:18

"And Joshua said, Roll great stones unto the mouth of the cave, and set men by it to keep them:" — Joshua 10:18 (ASV)

And Joshua said, Roll, etc. The enemy having been completely routed, Joshua is now free and, as it were, at leisure to inflict punishment on the kings. In considering this, the divine command must always be kept in view. Without this, it would demonstrate boundless arrogance and barbarous atrocity to trample on the necks of kings and hang up their dead bodies on gibbets. It is certain that they had recently been raised by divine agency to a sacred dignity and placed on a royal throne. It would therefore have been contrary to the feelings of humanity to exult in their ignominy, had God not so ordered it. But as such was His pleasure, we ought to acquiesce in His decision, without presuming to inquire why He was so severe.

At the same time, we must remember, as I previously hinted: first, that all, from the least to the greatest, were deserving of death because their iniquity had reached the highest pitch, and the kings, as more criminal than the others, deserved severer punishment; and secondly, that it was expedient to give an example of inexorable rigor in the persons of the kings, whom the people, from a perverse affectation of clemency, might have been too much disposed to pardon. It was the will of God that all should be destroyed, and He had imposed the execution of this sentence on His people. Had He not strongly stimulated them to perform it, they might have found specious pretexts for granting pardon. But a mercy that impairs the authority of God at the will of man is detestable. Now, however, when regal honor is not spared, all basis for showing mercy to the common people is removed.

By this instance, the Lord shows us the great interest He takes in His elect people. For it was an instance of rare condescension to place kings under their feet and allow them to scornfully triumph over their dignity, as if they had been petty robbers; as it is said in the Psalm, A two-edged sword is in their hand to execute vengeance on the nations, to bind their kings with fetters, and their nobles with chains of iron; to execute the judgment written: this honor have all the saints. (Psalms 149:6–9)

That fearful sight also had the effect of striking terror, so as to prevent the Israelites from imitating the ways of nations whose crimes they had seen so severely punished. Accordingly, we repeatedly find in the books of Moses this warning: You have seen how God took vengeance on the nations who were in the land of Canaan before you.

Beware, therefore, of provoking the wrath of your God by their perverse doings. In short, so that God might be worshipped with greater sanctity, He ordered the land to be purged of all pollutions. And as the inhabitants had been excessively wicked, He willed that His curse should rest upon them in a new and unusual manner.