In his zeal for the children of Israel and Judah When Joshua and the princes made a league with the Gibeonites, the people were greatly offended with them, as appears, Joshua chap. 9. Whatever the pretences of this resentment might be, the true reason seems sufficiently apparent; they were, by this league, deprived of the lands and spoils of the Gibeonites. Did these reasons cease in the days of Saul? Or rather, did they not still subsist, and with more force, in proportion as the people of Israel and their wants increased, in a narrow land? But however this may be, why did Saul slay them? The text plainly saith, that he did it in his zeal for the children of Israel and Judah. But the question still returns: How could the destroying these poor people manifest his zeal for Israel and Judah? There is seemingly but one imaginable way how this could be done. The Gibeonites had one city in the tribe of Judah, and three in Benjamin; and when they were destroyed out of these cities, who could pretend any right to them but Israel (that is, Benjamin) and Judah? So that Saul destroyed the Gibeonites, as the most obliging thing he could do for his people. See Delaney.
Related Commentaries of 2 Samuel 21:2
2 Samuel 21:2
2 And the king called the Gibeonites, and said unto them; (now the Gibeonites were not of the children of Israel, but of the remnant of the Amorites; and the children of Israel had sworn unto them: and Saul sought to slay them in his zeal to the children of Israel and Judah.)