Dostoevsky isn't simply warning against dishonesty—he's identifying self-deception as the root sin from which all other moral failures branch. When you lie to others, at least you're still accountable to your own conscience; but when you convince yourself that a selfish choice was actually noble, or that you had no choice when you did, you've poisoned the very instrument meant to guide you. You see this in the person who tells themselves they're "just being realistic" when they've actually surrendered their ambitions, or who insists they "had to" hurt someone when they merely chose the easier path. Once you've locked yourself in that cell, no external correction can reach you—which is precisely what terrified Dostoevsky most.