You find it again in the laws of the Nazir, who by vowing to abstain from wine and other legitimate pleasures has sinned against his soul (Num. 6:11). The Ha’amek Davar explains that the Nazir has sinned against his soul “by trying to be holier than he was capable of”.
So in the Korach story, the henchmen “who strove to attain a sanctity that was beyond them were guilty of sinning against their own souls, against themselves.”
You cannot put on the guise of great piety if it really isn’t you. Showing off how religious you are only shows up your self-pride or stupidity or both. The best avenue to piety is humility. Acknowledge how small you are in the scheme of things, how unworthy you are of God’s blessings, and let God do the rest. If He thinks you are right for it, He will help you towards religiosity.
Don’t parade your frumkeit and make a fool of yourself. If piety is going to happen, it will.