"Now Korah took" (Numbers 16:1). This is what Scripture says: "A brother offended is harder to win than a fortified city, and contentions are like the bar of a castle" (Proverbs 18:19). "A brother offended" — this is Korah, who quarreled against Moses, and rebelled, and sank down from the honor that had been in his hand. And the word "offended" (nifsha') means nothing other than rebellion, as it is said, "The king of Moab rebelled (pasha') against me" (II Kings 3:7); and it says, "Then Libnah rebelled (tifsha')" (II Kings 8:22). "And contentions are like the bar of a castle" (Proverbs 18:19) — for he quarreled against the Holy One, blessed be He, and against Moses.
Korah the Offended Brother Who Rebelled Against Moses and God
Midrash Tanchuma Buber, Korach 1:1
ויקח קרח (במדבר טז א). זש"ה אח נפשע מקרית עוז ומדינים כבריח ארמון (משלי יח יט), אח נפשע זה קרח, שחלק כנגד משה, ומרד וירד מכבוד שהיה בידו, ואין נפשע אלא מרידה, שנאמר מלך מואב פשע בי (מ"ב ג ז), ואומר אז תפשע לבנה (שם ח כב), ומדינים כבריה ארמון (שם), שחלק על הקב"ה ועל משה.