The Immutability of God is a necessary concomitant of His aseity. It is that perfection of God by which He is devoid of all change, not only in His Being, but also in His perfections, and in His purposes and promises. In virtue of this attribute He is exalted above all becoming, and is free from all accession or diminution and from all growth or decay in His Being or perfections. His knowledge and plans, His moral principles and volitions remain forever the same. Even reason teaches us that no change is possible in God, since a change is either for better or for worse. But in God, as the absolute Perfection, improvement and deterioration are both equally impossible. This immutability of God is clearly taught in such passages of Scripture as Ex. 3: 14; Ps. 102: 26-28; Isa. 41: 4; 48: 12; Mal. 3: 6; Rom. 1: 23; Heb. 1: 11,12; Jas. 1: 17.
Berkhof, Louis. Systematic Theology (p. 46). . Kindle Edition.
James Dolezal argues that if God is immutable in ethics, then this requires a metaphysical explanation:
Divine immutability enjoys more explicit biblical affirmation than doctrines such as divine aseity and infinity.44 Many of the supporting passages tend to focus on the constancy and faithfulness of God to do what he has promised to do, that is, upon his ethical immutability. Nevertheless, even ethical immutability requires an ontological explanation rooted in the very being and essence of God.
Dolezal, James E.. God without Parts: Divine Simplicity and the Metaphysics of God’s Absoluteness (pp. 81-82). Pickwick Publications, An Imprint of Wipf and Stock Publishers. Kindle Edition.
But immutability of ethics can be its own consideration. God is consistent in character. Just because a human being has the ability to murder another human being, we do not require a metaphysical explanation as to why he doesn’t.
From Vincent Cheung’s commentary on Malachi:
God first reminds the hearers of his immutability, saying, “I am the Lord, I change not” (v. 6). God’s attributes remains the same, and they will never change. He is not subject to any external influence, and he is eternal so that there is no before or after in his being, so that he does not change. His omniscience implies that he has no succession of thoughts, and therefore he does not change his mind. His knowledge and decisions eternally exist in his mind, and are not subject to alteration. Since he knows all, he does not gain knowledge, and nothing surprises him. Since he is eternally immutable and comprehensively perfect, he never becomes better or worse.
Cheung contradicts himself in the first sentence. God is immutable, meaning God has no “before’s and after’s” yet he is “reminding” “hearers”. Those sound like actions, in time, with “before’s” and “after’s”. Cheung seems not to be self-aware as to how the context of Malachi contradicts his claims about the meaning of the text.