Albert Barnes Commentary 2 Kings 11:20

"So all the people of the land rejoiced, and the city was quiet. And Athaliah they had slain with the sword at the king`s house." — 2 Kings 11:20 (ASV)

They slew Athaliah with the sword - This is one of the many small repetitions characteristic of the writer's style, which generally contains some small detail not mentioned before (compare 2 Kings 11:16).