The sefer She'eiris Menachem quotes from the Shem m'Shmuel (the same idea is quoted in other seforim as well) that a person who is blind in one eye is patur from the mitzvah of re'iya on the shalosh regalim because a person who comes to the beis hamikdash needs to see the world with two eyes: one eye focused on the gadlus of Hashem, and one eye focused on his own person shortcomings. A person who sees with only one eye, i.e. someone who sees and appreciates the gadlus of Hashem but is blind to their own faults, will not benefit from coming.
Rashi writes about Korach that "eino hit'aso," his eye deceived him. Korach saw the world with only one eye -- he saw the gadlus in ruchniyus that Aharon achieved, and maybe even was sincere in his desire for that same level of closeness with Hashem. "Boker v'yoda Hashem es asher lo" -- Rashi explains that Moshe told Korach that just like Hashem created boundaries in nature between morning and night, so too, Hashem created boundaries in ruchniyus and not everyone could be kohen gadol. The Emunas Moshe (Rebbe of Aleksander) explains that Korach was a big ma'amin. Who cares if there are rules in teva that way? A tzadik can rise l'ma'alah min ha'teva! Korach saw with his one eye that in ruchniyus everything is possible; Hashem has no boundaries. Korach had 20/20 vision in that eye and got it right. But when it came to seeing with his other eye, the eye that should have seen his own faults, he was completely blind. He failed to see that he was not Aharon; he lacked the lofty traits necessary to rise to the madreiga to which he aspired.