רַבִּי אֶלְעָזָר הַמּוֹדָעִי אוֹמֵר, הַמְחַלֵּל אֶת הַקָּדָשִׁים, וְהַמְבַזֶּה אֶת הַמּוֹעֲדוֹת, וְהַמַּלְבִּין פְּנֵי חֲבֵרוֹ בָרַבִּים, וְהַמֵּפֵר בְּרִיתוֹ שֶׁל אַבְרָהָם אָבִינוּ עָלָיו הַשָּׁלוֹם, וְהַמְגַלֶּה פָנִים בַּתּוֹרָה שֶׁלֹּא כַהֲלָכָה, אַף עַל פִּי שֶׁיֵּשׁ בְּיָדוֹ תוֹרָה וּמַעֲשִׂים טוֹבִים, אֵין לוֹ חֵלֶק לָעוֹלָם הַבָּא:
Rabbi Elazar of Modiin said: one who profanes sacred things, and one who despises the festivals, and one who causes his fellow’s face to blush in public, and one who annuls the covenant of our father Abraham, may he rest in peace, and he who is contemptuous towards the Torah, even though he has to his credit [knowledge of the] Torah and good deeds, he has not a share in the world to come.
Pirkei Avos, is saying that even if someone learns lots of Torah and does good things, but they embarrass someone in public, they have no share in the Olam Haba (the world to come). Bava Kammah speaks unto this mishnah;
The Gemara relates that the tanna who recited mishnayot and baraitot in the study hall taught a baraita before Rav Naḥman bar Yitzḥak: Anyone who humiliates another in public, it is as though he were spilling blood. Rav Naḥman bar Yitzḥak said to him: You have spoken well, as we see that after the humiliated person blushes, the red leaves his face and pallor comes in its place, which is tantamount to spilling his blood. Abaye said to Rav Dimi: In the West, i.e., Eretz Yisrael, with regard to what mitzva are they particularly vigilant? Rav Dimi said to him: They are vigilant in refraining from humiliating others, as Rabbi Ḥanina says: Everyone descends to Gehenna except for three.
What Bava Kammah is adding is that not only does one not have a share in the world to come, they also transgress one of the aseres hadibros, the ten commandments, which is to not murder. Because the Gemara says that embarrasing someone in public is like murdering them. The Gemara continues;
The Gemara asks: Does it enter your mind that everyone descends to Gehenna? Rather, say: Anyone who descends to Gehenna ultimately ascends, except for three who descend and do not ascend, and these are they: One who engages in intercourse with a married woman, as this transgression is a serious offense against both G-d and a person; and one who humiliates another in public; and one who calls another a derogatory name.
What they are adding here, is that not only do they transgress murder, and not go to Olam Haba, they don't go to Olam Haba because they one of 3 types of people who do not leave Gehenom (the underworld) after being banished there. Along with people who call people derogatory names, and one who sleeps with a married woman.
קרי עליה ושם דרך פי' כי זה חשב כי גדול ההפסד למלבין פני חברו ברבים מן השכר והתועלת שיהא לו בלימוד אותו דבר אפי' לא ידע אותו לעולם כ"ש שהי' יכול לראו' לאחרי כן:
Say onto him and there through even because he thought that the loss to whiten his friend's face is greater than the reward and benefit he will have in learning the same thing, although he did not know [it still applies]; 'It is possible to see after the fact.'
The Ritva, a 12th century commentator on the Gemara and a rishon, is adding to the Gemara what Pirkei Avos is saying and more, that his Torah learning and good deeds don't counteract this, even if they did not know. But one should not forget you can always do teshuva. But to do teshuva for a sin being adam l'chaverio, from one to his friend, to receive forgiveness you must ask the person for forgiveness (there is more but that is for another time).