"this abomination has been done in your midst": to include converts and freed slaves.
(Devarim 13:16) "Smite shall you smite": Whence is it derived that if you cannot put them (the inhabitants of a condemned city) to death by the specified penalty, you may do so by any of the death penalties of the Torah, whether of greater or lesser severity, and not leave it unpunished? From "Smite shall you smite" (in any event).