Rashi, at the beginning of the parashah writes, "פרשה זו יפה נדרשת במדרש רבי תנחומא, This parashah is taught beautifully in the Midrash Tanchuma." It is rare for Rashi to provide references. What is Rashi telling us?
The Pri Megadim (תיבת גומא עה"ת ד"ה קהלת), writes, "אומרים הלצה, people say a cute hint to explain this Rashi. It states (Mishlei 15:23), ודבר בעתו מה טוב, 'How good is a word in its time.' When it isn't the right time, it isn't nice. For example, during Pesach, it isn't the right time to discuss hilchos Succos or hilchos Yom Kippur. Rashi writes, פרשה זו יפה נדרשת, it is always the right time to speak about parashas Korach. This is because machlokes can happen at any time, so it is always appropriate to discuss this topic."
The Rambam (in a letter) describes the severity of machlokes. He writes, "Don't contaminate your souls with machlokes that destroy the body, neshamah, and wealth. I saw families perish; cities destroyed, communities dispersed, chassidim lost, honored people disgraced, all because of machlokes. The nevi'im foretold prophecies, and scholars spoke wisdom regarding the severity of machlokes, but even they haven't fully described just how bad it is. Therefore, hate it, run away from it, and keep away from all those who love machlokes, lest you be punished with them."
The Shlah (Shaar HaOsiyos אות ב' בריות, י"ד, ט"ז) 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 lowest Yid would certainly be moser nefesh rather than worship avodah zarah. So how could a person not be moser nefesh to avoid machlokes, for it is worse than avodah zarah! Even a drop of machlokes is too much! One spark of machlokes can create a fire that destroys everything."
This Shlah is a chiddush. Machlokes is worse than avodah zarah! The proof is in the following Midrash. The Midrash (Yalkut Shimoni 218) states: "[Nearly] everyone in Achav’s generation worshiped idols, yet they succeeded in their battles. This is because they didn’t speak lashon hara. On the other hand, in David HaMelech’s generation, even young children knew much Torah…but went to war and lost because there was lashon hara." The Shlah quotes this Midrash as a source that machlokes and lashon hara are worse than avodah zarah.
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. This is another source that 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 this choice. He shouted, “How can Shimon be our chazan? He is a baal aveirah!” Some sided with the Rav, while others wanted to hire Shimon. 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 HaMikdosh is preferred over a machlokes in klal Yisrael."
The Chasam Sofer zt'l explains that Aharon HaKohen agreed with the nation and helped them make the egel because he understood that if he refused, there would be machlokes. Aharon preferred the sin of avodah zarah over the even greater sin of machlokes.
We are surprised that machlokes is worse than avodah zarah. But that is what the Torah tells us, so we must be vigilant to avoid machlokes. We add that every machlokes contains an element of disbelief in Hashem. If one believed in Hashem, he wouldn’t stir up a machlokes. Rebbe Bunim of Pshischa zt'l teaches, "Emunah means that one believes that he has everything he needs, and if he needs more, Hashem will give him more. So, there is no reason to be jealous [or to fight to get more]. If you suffer from jealousy, work to remove it from your heart with all your strength and attach yourself to the emunah in Hashem's hashgachah pratis" (Beis Yaakov, Korach).
On a similar note, the Divrei Yisrael zt'l said that Korach's error was ויקח קרח, that he tried to take positions on his own. He thought it was in his hands to accomplish his desires and become the kohen gadol. But one doesn’t take on his own. Hashem gives, we receive, and everything is Hashem's decision.