John Gill Commentary 2 Kings 23:6

"And he brought out the Asherah from the house of Jehovah, without Jerusalem, unto the brook Kidron, and burned it at the brook Kidron, and beat it to dust, and cast the dust thereof upon the graves of the common people." — 2 Kings 23:6 (ASV)

And he brought out the grove from the house of the Lord Not a real grove of trees, but a carved one, as some think; or rather the image of the grove, (2 Kings 21:7) that is, the idol Ashtoreth, or Astarte, which was set up there; so Theodoret says; some interpreters call it Astoreth, the name of Venus, whom they call Astarte: this Josiah ordered to be brought without Jerusalem, unto the brook Kidron, and burnt it at the brook Kidron; the black brook, where the filth of the sacrifices was carried.

He stamped it small to powder, as Moses did the golden calf, and cast the powder thereof upon the graves of the children of the people; the common people, see (Jeremiah 26:23) or rather on the graves of the worshippers of idols, as it seems from (2 Chronicles 34:4) the Targum is, ``on the graves of the children of Galia,'' which, Kimchi says, is the name of an idol.

This was done partly in contempt of the idol, groves being, according to law, impure; and partly to the reproach of the deceased, and the memory of them, for their idolatry, and to deter from it those that survived them.