Christians actually believe that good is defined as that which is consistent with God's nature. Therefore God is neither arbitrary nor unjust, nor is he bound to an external law that would negate his power. The disproof from the nature of God results from an imperfect understanding of what the various characteristics of God really mean. Omnipotence does not mean the ability to do what is logically impossible. Further God's eternal nature is not that he always existed, but that he exists outside of time itself.
Time is created.
God's infinite love and justice are not contradictory. Rather God works in consistency to his loving-justice at all times. But just as the consistent application of the sun's nature can soften wax or harden clay, the consistent application of God's nature can result in wrath or mercy based on the response of his creation to his consistent loving-justice. The disproof from the nature of humanity first assumes that humanity has free will. At best humanity has limited free will, for I can not will myself to break the laws of nature.