They have deeply corrupted themselves - Literally, “they have gone deep, they are corrupted.” They have deeply immersed themselves in wickedness; have gone to the greatest depth they could, in it; they are sunk in it, so that they could hardly be extricated from it; and this, of their own deliberate intent; they contrived it deeply, hiding themselves, as they hoped, from God.
As in, the days of Gibeah - When Benjamin espoused the cause of “the children of Belial” who had worked such horrible brutishness in Gibeah toward the concubine of the Levite. This they maintained with such obstinacy, that, through God’s judgment, the whole tribe perished, except six hundred men. Deeply they must have already corrupted themselves, who supported such guilt. Such corruption and such obstinacy was their’s still.
Therefore “he will remember their iniquity.” God seemed for a time, as if He overlooked the guilt of Benjamin in the days of Gibeah, for at first He allowed them to be even victorious over Israel, yet in the end, they were punished, almost to extermination, and Gibeah was destroyed. So now, although He bore long with Ephraim, He would, in the end show that He remembered all by visiting all.