Saul sought to slay them, i. e. he sought occasions and pretences how he might cut them off with some colour of law or justice, diligently searching out and aggravating their faults, and punishing them worse than they deserved; oppressing them with excessive labours, and openly killing some of them, and intending by degrees to wear them out. In his zeal to the children of Israel and Judah; conceiving, or rather pretending, that it was not for the honour, nor comfort, nor advantage of God's people, to nourish any of that viperous broad in their bosoms; and that howsoever Joshua and the princes which then were, had by their fraud been drawn into an oath to preserve them, Joshua 9:15, yet in truth that oath was contrary to God's command, Exodus 23:32 Deuteronomy 7:2, and therefore (as he thought) not to be observed. This was his pretence. But how little zeal he had for God, or for the public good of his people, is evident by the whole course of his life; and therefore it is more than probable he had some particular motive or design in the case; either because some of them had highly provoked him, for whose sake he would be revenged of the whole race; or because, they being cut off, their estates might be forfeited to the crown; or for some other reason now unknown.
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.)