Or HaChaim on Numbers 5:6/אור החיים על במדבר ה׳:ו׳

1 איש ..כי יעשו מכל חטאת האדם, "anyone committing any of the sins people are liable to commit, etc." The Torah uses the expression למעול instead of the customary ומעלו מעל, "and have committed a trespass;" this is explained in Baba Kama 110 where the sages say that the verse speaks about a case of someone who perjured himself after having denied being in possession of money belonging to his neighbour. Although the Torah had already dealt with such a situation in Parshat Vayikra it repeats it here as there are numerous details of this legislation which had not yet been revealed. Amongst other details, our verse informs us that the person in question is deemed to have committed his sin at the time when he first denied being in possession of that money, not at the time when he denied it on oath. The very denial is considered sinful as it leads to the person having to swear an oath if his accuser takes him to court. If the accused is not taken to court he is able to make amends for his denial at any time. The verse says: כי יעשו מכל חטאת האדם to inform us that the moment one does so one is considered as about to also trespass against G'd, i.e. to render a false oath.

2ואשמה הנפש ההיא, and that soul will be considered guilty. Inasmuch as that person is guilty of perjury he has caused damage to his very soul. The Torah suddenly switched to the use of the singular although previously it had spoken about people (pl) committing sins. Afterwards, when the Torah discusses the sinner confessing his sin it again uses the plural, i.e. והתודו. You will note that the Torah did not write that the soul of the sinner i.e. נפשו is guilty, but it writes הנפש ההיא. This is a reference to the collective soul of the Jewish people which has become tarnished through one of its members committing perjury. You may do well to read what we have written in our commentary on Leviticus 7,20-21 about similar constructions. Inasmuch as the word ההיא is superfluous this is a hint that the Torah speaks about a soul other than merely that of the sinner in question. It is worth reading what the Zohar on Parshat Acharey Mot 67 has to say about Isaiah 26,9.