The verse gives certain additional particulars of David’s conquest of Edom (marginal references). Joab was left, or sent, to complete the subjugation of the country, with orders to exterminate all the grown male inhabitants. It was not very often that David acted with any extreme severity in his wars; but he may have considered himself justified by policy, as he certainly was by the letter of the Law Deuteronomy 20:13, in adopting this fierce course against Edom.
Was in Edom - Or, according to another reading, “smote” Edom.
The slain - Probably the Israelites who had fallen in the strnggle. Translate, “when ... Joab was gone up to bury the slain, and had smitten every male,” etc.