Charles Ellicott Commentary 2 Chronicles 30:17

"For there were many in the assembly that had not sanctified themselves: therefore the Levites had the charge of killing the passovers for every one that was not clean, to sanctify them unto Jehovah." — 2 Chronicles 30:17 (ASV)

There were many in the congregation that were not sanctified. — Compare 2 Chronicles 35:6; 2 Chronicles 35:10–11, where the Levites are again represented as doing the same work, but not as an exception. The precedent of Hezekiah’s Passover would seem to have become the rule. (Compare also Ezra 6:20.)

To sanctify them — that is, the lambs, which would have been ceremonially unclean if slain by unclean hands. (Compare 2 Chronicles 2:4, to dedicate it unto Him; the same verb.)