Now, God defines goodness, and thus what he is and does is ipso facto good. Whatever he is and whatever he does is good, which means that no standard of goodness external to God may be used to judged an act of God as good or evil. We derive the very definition of goodness from what God is and does.
As mentioned, we discover what is good or moral through the Scripture. And earlier it is said that the view saying that the definition of goodness is in a sense arbitrary cannot be dismissed. For example, it was good for Old Testament believers to be circumcised solely because God had commanded it. Therefore, it was good for an Old Testament believer to be circumcised, and evil for him not to be circumcised.
The definition of goodness is therefore "arbitrary," but only in the sense that God's will determines everything, including the standard of goodness. By arbitrary, therefore, we do not mean, "existing or coming about seemingly at random or by chance or as a capricious and unreasonable act of will," but rather something similar to, "not restrained or limited in the exercise of power: ruling by absolute authority" (Merriam-Webster's Collegiate Dictionary, Tenth Edition).
The doctrine of the simplicity of God dictates that we regard his attributes as one, which means that there can be no separation between his will and nature. All things, in this sense, are arbitrary but necessary, since there is no explanation to anything more final than to say that God has willed it so, and there is nothing prior to God's will that dictates or influences what he wills. He is love and he wills to be love; he wills to be love and he is love. God's will is the final explanation; there is no prior cause.
Therefore, for one to kill another is not inherently immoral, but is only so due to God's commandment, "You shall not murder." By the same token, it would have been immoral for Abraham to restrain from preparing Isaac for sacrifice, once God has commanded it to be done in another context, we would call it murder. If God had not stayed Abraham's hand, it would still have been good for him to have killed Isaac simply because God had commanded it. The justification for capital punishment is likewise derived. God has complete sovereignty over all creation, and whatever he commands is good by definition.