The word chet is "sin" and is used as such (and in related ways -- sin offering, guilt from sinning, cause others to sin etc.) over 200 times according to my Even-Shoshan. It seems to be connected to a word meaning "miss" as it is used in Shoftim 20:16. But in some cases, the first one being in Vayikra 14:49, the root belongs to a word that means "to purify." The Targum there translates it into the same Aramaic root as it would a word based in t-h-r.
When a word like "k-d-sh" is used to mean something other than "holy" there are commentators who explain both the linguistic/etymological ("separateness") and the stylistic (as a euphemism) but I was unable to find anyone in a Mikra'ot Gedolot who comments on that pasuk in Vayikra to explain why a word which means sin suddenly means "make pure."
Is there any commentary who explains the word and its use in this way?
s a side note (edit) if the word is used to mean "purify" then the Gematria of 18 makes much more sense.