“standing-stones.” Standing-stones were set up for various reasons, some of them being godly memorials, but here the context is pagan worship. Standing-stones would often be set up as part of the worship of pagan gods, and God has no tolerance for idols. They are harmful in many different ways and are to be destroyed. [For more on standing-stones, see commentary on Gen. 28:18. For more on idols being harmful, see commentary on Deuteronomy 7:5].
“the bronze serpent that Moses had made.” The bronze snake that Moses made would now be about 700 years old (cp. Num. 21:4-9).
“for in those days the children of Israel burned incense into smoke-perfume to it.” When the worship of Moses’ serpent began is not known. The idolatrous Judeans turned a priceless artifact of history into an idol, and that being the case, Hezekiah did the right thing and destroyed it. Too often believers are tricked into keeping things that have become idols, “protective amulets,” etc., just because those things have historical or family significance. But demons are attracted to the love, devotion and even forms of worship that some objects receive, and so believers must be on guard to keep historical pieces and heirlooms as just that, and not began to ascribe protective power, “luck” or any kind invisible power to them and thus turn them into idols, which only invites spiritual problems.