Chapter thirty of the Tanya instructs: "Be humble of spirit before every person" (Avot 4:10)—and it means every person, including the worst person you can imagine.
How is this possible? The Tanya's answer: "Do not judge your fellow until you have stood in his place" (Avot 2:4). The person who sins flagrantly may be in a situation you cannot fathom. His livelihood forces him into the marketplace all day, surrounded by temptation. His eyes see what your eyes have never seen. His evil inclination may burn like a baker's oven—a level of desire you have never experienced.
Rabbi Schneur Zalman is not excusing the sinner. He says explicitly that even such a person is still called "an utter evildoer" because he should have controlled himself despite the difficulty. Fear of God—knowing that God sees every action—should have been enough to restrain him. But—and this is the crucial pivot—the question is not whether he is wicked. The question is whether you have the right to feel superior.
The Tanya says: if you spend your days in study and rarely face severe temptation, your righteousness may be easier than his struggle. The person who battles a raging fire of desire and loses is still a more intense spiritual combatant than the person who never faced the fire at all. "Each person according to his place and rank" must examine whether his service of God is proportional to the difficulty of his personal test.
This teaching generates a paradox that the Tanya embraces: genuine humility before every person is compatible with recognizing that the other person may be wicked. You are not pretending that sin is acceptable. You are admitting that if you had that person's temperament, that person's circumstances, that person's level of temptation, you might have done worse. The comparison destroys arrogance without destroying moral standards.