עַל־דְּבַ֞ר אֲשֶׁ֨ר לֹא־קִדְּמ֤וּ אֶתְכֶם֙ בַּלֶּ֣חֶם וּבַמַּ֔יִם בַּדֶּ֖רֶךְ בְּצֵאתְכֶ֣ם מִמִּצְרָ֑יִם וַאֲשֶׁר֩ שָׂכַ֨ר עָלֶ֜יךָ אֶת־בִּלְעָ֣ם בֶּן־בְּע֗וֹר מִפְּת֛וֹר אֲרַ֥ם נַהֲרַ֖יִם לְקַֽלְלֶֽךָּ׃
וְלֹֽא־אָבָ֞ה ה׳ אלקיך לִשְׁמֹ֣עַ אֶל־בִּלְעָ֔ם... כִּ֥י אֲהֵֽבְךָ֖ ה׳ אלקיך
How can the pasuk juxtapose and equate something so trivial like not serving food with the sin of hiring Bilam to destroy Klal Yisrael? Not only does it put the two together, but it puts the not bringing out the food first! R' Baruch Mordechai Ezrachi suggests a thought experiment: if you wanted to get a Bilam to do teshuvah, what would you have him do? Keep shabbos? Eat kosher? Daven? R' Ezrachi says that even if he did all that, all you would have is a Bilam who davens, eats kosher, and keeps Shabbos -- the same rotten person, just one who does a few mitzvos. The way to change the core of the person and make Bilam not Bilam anymore is to have him do chessed like serving food to a stranger, something that will change his midos. The אֲשֶׁ֨ר לֹא־קִדְּמ֤וּ אֶתְכֶם֙ בַּלֶּ֣חֶם וּבַמַּ֔יִם is the siman that at core, Amon and Moav were rotten people. What starts with a trivial slight ends up with אֲשֶׁר֩ שָׂכַ֨ר עָלֶ֜יךָ אֶת־בִּלְעָ֣ם בֶּן־בְּע֗וֹר . It's just a matter of degree, not a qualitatively different sin.
We're supposed to be doing teshuvah during Elul. The Chayei Adam (143) writes at length about how during this time of year a person is supposed to add time to his learning, add to the $ he gives to tzedaka, adds to his zehirus in shemiras ha'mitzvos. I think what you learn from R' BM"E is that Reb Ploni can do all that -- he can learn an extra hour, he can wake up at dawn for slichos, he can write checks to a dozen different charities -- and yet he can still be the same Reb Ploni, just with a few extra mitzvos under his belt. The goal of teshuvah has to be to not be the same Reb Ploni.
Taz explains that the two different reasons given for our rejecting Amoni and Moavi, 1) עַל־דְּבַ֞ר אֲשֶׁ֨ר לֹא־קִדְּמ֤וּ אֶתְכֶם֙ בַּלֶּ֣חֶם וּבַמַּ֔יִם and 2) אֲשֶׁר֩ שָׂכַ֨ר עָלֶ֜יךָ אֶת־בִּלְעָ֣ם בֶּן־בְּע֗וֹר are hah b'hah talya. Why should Amon and Moav be held accountable for not offering food and drink more than any other nation? (see Ramban!) Taz quotes the pasuk in Mishlei 25:21 אִם־רָעֵ֣ב שׂ֖נַאֲךָ הַאֲכִלֵ֣הוּ לָ֑חֶם וְאִם־צָ֜מֵ֗א הַשְׁקֵ֥הוּ מָֽיִם, that there is a mitzvah to extend hospitality davka to your enemy, the person who you like least, because that's how to overcome your yetzer ha'ra. Similarly, the gemara (BM 32) writes that even though unloading a burden from an animal is a bigger mitzvah than loading, if the person who needs help loading is your enemy, then that mitzvah comes first. And we learned from the Netziv earlier this year, the way to solve a dispute is to do something good davka for the person who has harmed you. If it would have been anyone else, any other nation, the fact that we were not greeted with a welcome parade and a tray of cake and cookies would not have been that big a deal. But since שָׂכַ֨ר עָלֶ֜יךָ אֶת־בִּלְעָ֣ם בֶּן־בְּע֗וֹר, proving their hate, Amon and Moav had an even greater obligation to try to do good. (Amazing that this obligation to break one's yetzer extends even to a ben Noach. ) Doing what we like least, or doing something good for someone we like least, is transformative in a way that just doing more of what we are already good at is not.
The argument against Amon and Moav is m'mah nafshach: if you hate Bnei Yisrael, all the more reason you owe them the favor of a proper greeting with food and drink. And if you don't really hate them because you know that G-d loves them, כִּ֥י אֲהֵֽבְךָ֖ ה׳ אלקיך, and it would be wrong to hate that which G-d loves, then why did you go out and hire Bilam to curse them?