Albert Barnes Commentary 2 Kings 18:4

"He removed the high places, and brake the pillars, and cut down the Asherah: and he brake in pieces the brazen serpent that Moses had made; for unto those days the children of Israel did burn incense to it; and he called it Nehushtan." — 2 Kings 18:4 (ASV)

He removed the high places - This religious reformation was carried out in a violent and tumultuous manner. Although forbidden in the Law (Deuteronomy 12:2–4, 11-14), the “high places” had in practice received the sanction of Samuel (1 Samuel 7:10; 1 Samuel 9:12–14), David (2 Samuel 15:32), Solomon (1 Kings 3:4), and others, and had long been the favorite resorts for most of the people (see 1 Kings 3:2 note). They were the rural centers for the worship of Yahweh, serving the role of the later synagogues, and until now had been tolerated—or rather, regarded as legitimate—even by the best kings. Hezekiah’s desecration of these time-honored sanctuaries must have been a rude shock to many people, and indications of the popular discontent can be traced in the appeal of Rabshakeh (2 Kings 18:22) and in the strength of the reaction under Manasseh (2 Kings 21:2–9; 2 Chronicles 33:3–17).

The bronze serpent - Its history from the time it was set up until Hezekiah’s reformation is a blank. The present passage favors the supposition that it had been brought by Solomon from Gibeon and placed in the temple, as it implies a long-continued worship of the serpent by the Israelites generally, and not merely a recent worship by the people of Judah.

And he called it Nehushtan - Rather, “And it was called Nehushtan.” The people called it not “the serpent” (נחשׁ nāchāsh), but “the brass” or “the brass thing” (נחשׁתן n^echûshtān). They probably did not like to call it “the serpent” because of the dark associations attached to that reptile (Genesis 3:1–15; Isaiah 27:1; Psalms 91:13; etc.).