They slew Athaliah with the sword - This is one of the many little repetitions which mark the manner of the writer, and which generally contain some little point which has not been mentioned before (compare 2 Kings 11:16).
Related Commentaries of 2 Kings 11:20
2 Kings 11:20
20 And all the people of the land rejoiced, and the city was in quiet: and they slew Athaliah with the sword beside the king's house.