Hezekiah destroyed both the other images (theirs) as well as the brazen serpent God Himself commanded Moses to make.
Both idols and an icon.
For one, it looks like for censing the icon (made by Moses according to His commandment) but not according to His instruction concerning it. The other images God did not command them to make having made these things unto themselves.
I dont' see why Hezekiah would go into the most holy place where the golden censer and ark of the covenant would be as was ordained to destroy it. God commanded that to be made also, the two figures as well which were made facing one another whose faces were covered by their outstretched wings. These were ordained this way, as God told them he would meet with them both between the cherubs (and above them).
It shows us all, graven images (even idols) being destroyed by Hezekiah, and an icon made by Moses being broken to pieces, and elsewhere concerning the ark Uzza having reached out his hand to the ark was killed, (and by God) for doing that.
There are other instances, for example, Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the LORD, which he commanded them not. And there went out fire from the LORD, and devoured them, and they died before the LORD.