As with every Parsha, the Noam Elimelech says a lot of fascinating things on Parsha Korach. Here is one of them:
The Medrash Shocher Tov says: "Why is תּוֹרַת ד׳ תְּמִימָה? (Why is the Torah of Hashem complete?) Because מְשִׁיבַת נָפֶשׁ, it restores souls". This Medrash is a really great wonder. What is this supposed to mean? The Mekubalim write that Korach was a gilgul (reincarnation) of Kayin, therefore, Korach was swallowed up in the earth, as a tikun for the earth that opened up for the blood of Kayin's brother, Hevel. That's why Kayin was reincarnated in Korach. But, why couldn't the tikun be done through Kayin himself at that time? Because Hashem created gilgulim in order to make the Torah complete, for if it wasn't so (and there was no Korach), we would be missing the posuk of וַיִּקַּח קֹרַח in the Torah. Now we can explain the words of the Medrash. Why is the Torah complete? Because it returns souls; through Hashem creating gilgulim, the Torah is complete.