2 Samuel 21:2
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.)
The Gibeonites were not Israelites but a remnant of the Amorites. Israel had sworn an oath to them to protect them, and that sworn league remained binding. The league was the one Joshua had made, and it bound later generations. Although they had obtained it by deception, Joshua made them servants as punishment, and their lasting presence as heathen meant Israel could not fully drive them out as God had intended.
Saul sought to slay and destroy them in his zeal for the children of Israel and Judah, violating and seeking to break the oath. He killed some of them and apparently planned to massacre the rest, acting from mistaken patriotism. The account stands as part of a detailed and faithful history of David's reign and its difficulties, which does not conceal the sins of even a great king. God holds His people collectively responsible for the transgressions of a few when the many allow the sin to continue. An offense by one person was charged to the whole family, so Saul's attempt to destroy the Gibeonites brought a duty to seek redress on behalf of those he wronged.
Saul's zeal for Israel and Judah did not excuse this oppression, for God holds His people accountable for wrongs against those outside their community. God observed this injustice, and it brought consequences upon the land. Their continued existence in David's time, with political rights and the ability to make a treaty with Israel's king, shows that their service was not chattel slavery. They remained personally free and possessed their own homes, lands, and families.