Why did he grind up Moshe's copper snake?
Radak (from Avodah Zarah 44a #1): He ground it up and cast [the particles] to the wind, for it was Asur b'Hana'ah.
Radak (from Avodah Zarah 44a #2): It belonged to Moshe. Others could not forbid benefit from it. He ground it up because people were sinning through it.
When did they start offering to it?
Radak: It is when Malchei Yehudah sinned and Yisrael strayed after idolatry. Since it says "v'Ra'ah Oso va'Chai" (Bamidbar 21:8), people thought that it is a good intermediary to serve.
Why didn't Asa and Yehoshafat destroy it? They destroyed all other idolatry!
Radak: They did not find people offering to it, so they left it for a remembrance of the miracle, just like the jar of Manna. In Chizkiyah's days they were serving it like idolatry. Even though for Tzadikim it recalled the miracle, it is better that the miracle be forgotten than for Yisraelim to err and serve it today or later 1 .
Radak citing Chulin 7a: Hashem caused that Asa and Yehoshafat not destroy it, to leave room for Chizkiyah to aggrandize himself through doing so.
Tosfos (Chulin 7a): They erred. They thought that since Hashem commanded to make it, it is forbidden to destroy it. Malbim - the Ran (Avodah Zarah 24b) says that Amora'im argue about whether or not one can forbid another's property via an action. Asa and Yehoshafat held that one cannot, therefore they did not destroy Moshe's snake. Chizkiyah holds that one can - this is why he put Achaz' Kelim into Genizah - therefore also Moshe's snake was forbidden, so he destroyed it.
Tzadikim would remember it, for the Torah discusses it, just it is a greater remembrance through seeing it. Chizkiyah had great reason to be concerned lest in the future, people return to serve idolatry, and serve it. He saw through Ru'ach ha'Kodesh that he will father evil children (Brachos 10a)! (PF)
Why did he call it "Nechushtan"?
Rashi: This is for disgrace. What power does it have? It is merely a copper snake! Radak - the superfluous final Nun belittles it, like the Yud in "ha'Aminun" (Shmuel II, 13:20), and the Vov-Nun at the end of Ishon and Shabboson.
Targum Yonasan: People (Radak - its worshippers) called it so 1 .
Surely they considered this an honorable name. (PF)
