Jamieson, Fausset and Brown on 2 Samuel 21:2

2. in his zeal to the children of Israel and Judah--Under pretense of a rigorous and faithful execution of the divine law regarding the extermination of the Canaanites, he set himself to expel or destroy those whom Joshua had been deceived into sparing. His real object seems to have been, that the possessions of the Gibeonites, being forfeited to the crown, might be divided among his own people (compare 1Sa 22:7). At all events, his proceeding against this people was in violation of a solemn oath, and involving national guilt. The famine was, in the wise and just retribution of Providence, made a national punishment, since the Hebrews either assisted in the massacre, or did not interpose to prevent it; since they neither endeavored to repair the wrong, nor expressed any horror of it; and since a general protracted chastisement might have been indispensable to inspire a proper respect and protection to the Gibeonite remnant that survived.