He is holy.
Now the question of where that standard came from, and whether it is something that exists independent of God or created by God, is a separate issue. Perhaps it is a mystery which we cannot give a definitive answer to at the present time. But that does not alter the validity of the argument presented above, about the immutable nature of the the character of God as revealed in holy scripture. “Good” is not “determined by what God does;” but there is an independent “standard of goodness” which God adheres to.
Throughout the scriptures God has commanded mankind to do good and be good. That presupposes a fixed, predetermined standard of goodness and righteousness that he expects mankind to adhere to—otherwise it would be impossible for man to comply with that expectation. If such a standard exists for man, does a different standard exist for God? If there are two different standards, how can “good” have any meaning for man or God? Such expectation would be meaningless unless the same standard was used by both. God’s commandment is, “Ye shall be holy; for I am holy” (Lev. 11:44–45; 19:2; 20:7, 26). This can only make sense if the same criteria or standard of holiness is used for both. And this applies equally to all the other character traits or attributes of God. Any observable difference could only be accounted for by man’s limited knowledge compared to God’s. God knows things that man does not; therefore some of God’s actions may appear to be morally inexplicable to man for that reason.
(The original version of the above video had poor sound and image quality; so I downloaded it to my PC, improved the sound and image quality, and uploaded it to my own YouTube channel. The original version can be seen here.)