Matthew Poole and continuators on 2 Kings 11:1

AND when Athaliah the mother of Ahaziah saw that her son was dead, she arose, and destroyed all the seed royal .

i. e. All of the Royal Family, left after those Slaughters, 2 Chron. 21. 2, 4. and 22. 1. and 2 King. 10. 13, 14. except one, as the next Verse limits and explains it. This she did, partly out of Rage at the Extirpation of her Family; resolving, that David's Family should not out-live hers; partly, from ambition and desire of Rule; for which many persons have Destroyed their nearest Relations: partly, from the zeal which she had for Idolatry, and the Worship of Baal which she intended to Establish; to which she knew the House of David were implacable Enemies. And partly, in her own Defence, that she might secure her self from Iehu's Fury, who was commanded by God, and resolved in himself, utterly to destroy all the branches of Ahab's Family, whereof she was one, Chap. 8. 18, 26. For had she not done this, she suspected, that either the King, or People of Iudah would have delivered her up to Iehu upon his Demand. Possibly those whom she slew were Ioram's Children by another Wife: Of which, see on the next Verse. And this was the fruit of Iehoshaphat's Marrying his Son to a Daughter of that Idolatrous and Wicked House of Ahab, even the Extirpation of all his Posterity, but one. And this Dreadfull Judgment God permitted, and Inflicted upon him and his, to shew how much he abhors all such sinful and unequal affinities.

Marginal references: 2 Chro. 22. 10.; Heb. seed of the Kingdom.