Why did Hashem make them disgraced and lowly?
Radak: Like you disgrace Me, I made you disgraced. You are partial to the nation, and do not rebuke them, so they will love you - I will make you disgraced and lowly in their eyes. This is lest people say, that false witnesses are lowly in the eyes of those who hired them (Sanhedrin 29a).
Malbim: Levi used to be honored in the eyes of the nation. They were like an angel, elevated and Mekudash. I made you disgraced in the eyes of the nation, and you are truly lowlier than the entire nation.
What do we learn from "k'Fi Asher Einchem Shomerim Es Derachai"?
Radak: You disgrace Me, and do not guard My way - Midah k'Neged Midah, I will make you disgraced and lowly.
Malbim: I lowered you from your level according to the amount that you do not guard My way. As you increased to descend from the level of Levi and Kehunah, you descended more in disgrace and lowliness.
How are they "Nosei Panim ba'Torah"?
Radak: You are partial to great people in a matter of Torah. It says "Lo Sakrivu Eleh la'Shem" (Vayikra 22:22), and when they bring a Korban Ba'al Mum, you fear to rebuke them, and say 'this Korban is Pasul.'
