Rashi says that Korach saw with ruach hakodesh (divine inspiration) that the prophet Samuel would descend from him. Yet Rashi also implies he had the bad traits of jealousy and arrogance.
How could he have ruach hakodesh if he had the bad traits of jealousy and arrogance?
it seems one must be clean of all bad traits before reaching this level as explained in the book Mesilas Yesharim
Tiferes Tzion says that even though Korach lacked the refinement of character to receive prophecy, Ruach Hakodesh still rested upon him because he lived in a holy environment that was conductive to Ruach Hakodesh.
I would propose - but have no source for it - that Korach's bad traits of jealousy and arrogance were a result of his prophesy.
In other words, Korach was a simple Jew happy to go along with Moshe's guidance until he saw - with Ruach HaKodesh - that his descendants would be great people.
At that point he became arrogant - he was the progenitor of a great lineage! Subsequently he became jealous; why shouldn't such an important person as himself be recognised as such.
By then he was no longer entitled to Ruach HaKodesh; we clearly see from his fatal bad decisions that he could no longer forsee upcoming events.
Perhaps Korach experienced his ruach hakodesh at the giving of the Torah during which even the lowest handmaid experienced a prophecy greater than that of Ezekiel? (Rashi on Shemot 15:2)
I'll try to add a thought without referring to a specific tradition.
Korach lacked refinement of character but how was any of the men Ruach Hakodesh ever revealed something without flaw? Moshe was not and he knew, which seems contributed to him being the most humble man on earth and particularly inclined to listen to G-d and follow suit. However Moshe killed an Egyptian overseer in a rage of fury. G-d still called Moshe to service in the desert. Years later Moshe disobeyed G-d by striking the rock in a rage of fury (same hidden flaw of character surfacing again) and in the aftermath Moshe was not allowed to enter Israel, still G-d did not stop using Moshe as leader of Yisrael while he lived.
Put very simply, I conclude from this that the prerequisite for G-d to approach a man and convey something to him is not perfection in the man (that's not ever been seen happening) but G-d deciding to do so. If G-d never covered the hidden sins before His face, as David prayed (well aware that properly, who may stand before G-d in a sacred place? He who is completely pure - that's no man), then there would be no G-d approaching a human, as things have come to stand, at all.
As a side note: No offense meant but "level" and "conductive" in this context both seem to sound indicative of inclinations to schools of thought that sound esoteric to me. I seem to find in the Torah that Ruach Hakodesh is not into esoterics but very opposed to any system of it.
