They did not destroy the nations of which the Lord spoke unto them.

“But they of whose iniquities this Psalm speaks, when they had entered into that temporal land of promise, "destroyed not the heathen, which the Lord commanded them" [Psalm 106:34]; "but were mingled among the heathen, and learned their works" [Psalm 106:35]. "Insomuch that they worshipped their idols, which became to them an offense" [Psalm 106:36]. Their not destroying them, but mingling with them, became to them an offense.”

“They did not destroy the nations,.... Here begins an account of their sins and provocations, after they were settled in the land of Canaan. They did not destroy the inhabitants of the land, of the seven nations; whose land was given to them as an inheritance, and of which the Canaanites were dispossessed for their sins, and to be destroyed. Concerning whom the Lord commanded them; that they should destroy them; the command is in Deu 7:1. God's commands are to be obeyed; they are neither to be added to, nor diminished from; his commands are transgressed and violated by sins of omission or commission; the Israelites might plead mercy, but this was no excuse to an express command: the same sin Saul was afterwards guilty of, with respect to one of these nations, Sa1 15:2. Those spiritual Canaanites, the sinful deeds of the body, are to be mortified, and not indulged and spared, Col 3:5.”

“Them. Commanding no idolatrous government to be left in the land, nor any Chanaanite to be permitted to live, who should make opposition. The law of extermination must probably be thus restricted. The Jews neglected these ordinances, and hence became so often infected with idolatry, (Berthier) even to the end of the republic, when many still adored the sun. (Josephus, Jewish Wars ii. 8.) (Exodus xxiii. 32., and Josue xiii. 13.)”

“The sins in Canaan: the failing to exterminate the idolatrous peoples and sharing in their idolatry. In Psa 106:34 the poet appeals to the command, frequently enjoined upon them from Exo 23:32. onwards, to extirpate the inhabitants of Canaan. Since they did not execute this command (vid., Jdg 1:1), that which it was intended to prevent came to pass: the heathen became to them a snare (mowqeesh), Exo 23:33; Exo 34:12; Deu 7:16. They intermarried with them, and fell into the Canaanitish custom in which the abominations of heathenism culminate, viz., the human sacrifice, which Jahve abhorreth (Deu 12:31), and only the demons (שׁדים, Deu 32:17) delight in. Thus then the land was defiled by blood-guiltiness (חנף, Num 35:33, cf. Isa 24:5; Isa 26:21), and they themselves became unclean (Eze 20:43) by the whoredom of idolatry. In Psa 106:40-43 the poet (as in Neh 9:26.) sketches the alternation of apostasy, captivity, redemption, and relapse which followed upon the possession of Canaan, and more especially that which characterized the period of the judges. God's "counsel" was to make Israel free and glorious, but they leaned upon themselves, following their own intentions (בּעצתם); wherefore they perished in their sins. The poet uses מכך (to sink down, fall away) instead of the נמק (to moulder, rot) of the primary passage, Lev 26:39, retained in Eze 24:23; Eze 33:10, which is no blunder (Hitzig), but a deliberate change.”