It’s well known that in ancient Judaism, as in numerous other cultures, there were concepts of both ritual and moral pollution. These overlap, but can also be distinguished: someone who touches a corpse in order to bury them is ritually unclean, but is certainly not sinful; on the contrary, they are pious (see Tobit 1:16-18). Someone who acts sinfully, even without contracting ritual uncleanness, may bring moral pollution to the land.
Anyway, given that we are accustomed, in New Testament studies, to consider these things in relation to Judaism, I was interested to come across the themes of moral pollution and purification in Plato’s Laws. Murder results in the offender being legally unclean, and in need of purification (as well as needing to provide recompense if it was intentional). A number of different scenarios are considered:
if he has killed someone and his hands are polluted by murder, he must depart to a place in another country and live there in exile for a year (Laws, book 9, 864e; Cooper, 1522)
[In accidental murder during military training] the offender shall be free of pollution when he has been purified in accordance with the relevant law from Delphi (Laws, book 9, 865b; Cooper, 1523)
All doctors, if their patient dies as an unintended result of their treatment, are to be free of pollution according to law. (Laws, book 9, 865b; Cooper, 1523)
[If someone accidentally kills a slave and fails to indemnify the master] he must resort to greater and more numerous purifications than those who have killed in contests; and such expounders as are chosen by the oracle are to be in charge of these purifications (Laws, book 9, 865c; Cooper, 1523)
If he kills a slave of his own, let him purify himself, and be quit of the murder according to law. (Laws, book 9, 865d; Cooper, 1523)
Paul frequently uses the terminology of ‘impurity’ in his vice lists, and I think that Jewish uses of the term and the concept are most fruitful for considering what Paul means – but it is worth keeping in mind that his Gentile hearers may well have had some sense/s of what it meant to be ‘polluted’ from their own cultures.