Job speaks a new argument against the friends: they show God partiality, that is, they have a bias to bend the truth in order to defend God. Throughout scripture, God declares you are not to show anyone partiality. Therefore, we should not try to lie about God or find a loophole to ignore what is going on in the world. We should speak the truth. In order to speak rightly about God, we look to the scriptures which reveal Him to us. We don't need to make excuses for God's judgment when that judgment is made perfect in Christ on the cross where there is Job's (and our) deliverance from evil.