The Shlah (Shaar HaOsiyos ז"ט ,ד"י ,בריות 'ב אות) writes, "Why do I need to elaborate on the ugliness of machlokes? All sefarim are filled with this subject! The sin of machlokes is worse than avodah zarah. The simplest Yid would certainly be moser nefesh rather than worship avodah zarah. So how could a person not be moser nefesh to avoid machlokes, which is worse than avodah zarah! Even a drop of machlokes is too much! One spark of machlokes can create a fire that destroys everything."
The Shlah's proves that machlokes is worse from the following Midrash: Yalkut Shimoni (218) states, "[Nearly] everyone in Achav’s generation worshiped idols, yet they succeeded in their battles because they didn’t speak lashon hara. In David HaMelech’s generation, even young children knew much Torah... but they went to war and lost, and this is because there was lashon hara."
The Shevet Mussar (37:22) points out that the manna fell almost every day in the desert. It even fell on the day Bnei Yisrael made the egel. But it didn’t fall on the day Korach made a machlokes because machlokes is worse than avodah zarah.
The Afrakasta d'Anya (165) writes that the community of Kempna hired "Shimon" to be their chazan. Reb Yosef Shmuel zt’l, the Rav of Kempna, was very against his appointment. He shouted, “How can Shimon be our chazan? He is a baal aveirah!” He was a severe baal aveirah. He didn't even keep kashrus, and who knows what else. Some sided with the Rav, while others wanted to hire Shimon, and the Kempna community was divided in two. Reb Yosef Shmuel sent a letter to Reb Yosef of Posen, seeking his counsel. The rav of Posen wrote back, "A tzelem [cross] in the Beis HaMikdash is preferred over a machlokes in klal Yisrael."
Many are surprised when they study parashas Ki Sisa and discover that Aharon HaKohen helped the nation make the egel. It states (Shemos 32:1) מן לרדת משה בשש כי העם וירא אלהים לנו עשה קום אליו ויאמרו אהרן על העם ויקהל ההר לפנינו ילכו אשר, "When the nation saw that Moshe was late in coming down from the mountain, the people gathered against Aharon and they said to him: 'Come on! Make us gods that will go before us...' ויאמר ובנתיכם בניכם נשיכם באזני אשר הזהב נזמי פרקו אהרן אלהם אלי והביאו, Aharon said to them, 'Remove the golden earrings that are on the ears of your wives, your sons, and your daughters and bring them [those earrings] to me.'" Rashi writes, "Aharon said to himself, 'The women and children are fond of their jewelry. Perhaps the matter will be delayed, and in the meantime, Moshe will arrive,'" but it appears that he helped them make the egel. Indeed, Moshe rebuked him for this. As it states (32:21) הזה העם לך עשה מה אהרן אל משה ויאמר גדלה חטאה עליו הבאת כי, "Moshe said to Aharon: 'What did this people do to you that you brought such a grave sin upon them?'" Aharon excused himself as it is written in the pasukim there, but this remains very surprising. How could it be that Aharon helped the nation make the egel?
The Chasam Sofer zt'l answers that Aharon understood that if he refused the nation and didn't help them make the egel, there would be machlokes. Aharon preferred the sin of avodah zarah over the even greater sin of machlokes.
