21, though true, states only the external or civil side of this commandment. That is the meaning of vs. 22. Anger in itself is not wrong. Anger is an attribute of God. But it should never be the kind that consumes the brother or turns into a grudge.
Christians are still sinners.
But they have a new man which must rule the old man. There are times when Christians, like Jesus, are rightfully angry. But the anger must not destroy or hurt the brother or become a grudge. 23-24 tell us that an unresolved situation with another human being is dangerous for the Christian. He must not live with a guilty conscience or offensive unforgiveness toward his neighbor.