I'm not assuming any such thing.
I'm only pointing out what He says! This is His warning, not mine.
Where are we told that God puts in an individual human being's hands the right to assess guilt and innocence and assign punishment? "Vengeance is Mine, sayeth the Lord: I will repay." Given that God knows that we need, in human affairs, to take action to protect ourselves, he lays out institutions and processes for trying to bring His will into the process of administering justice. In all of these processes the hand of the prosecutor and the accuser is stayed, in some way, by collective and unbiased decision making. This is one of God's great gifts to us.