There is subjective morality, in other words, a conscience. It is subjective because it is influenced by uncontrolled desires and cultural opinions. But there is only one God and only one morality that is objective, that is, his morality. God has a purpose for humans, and sometimes short-sightedness prevents some humans from seeing it.
Human-based laws can be changed by humans because of greed, fear of death or injury, cultural values, vengeance, desire for power, or just plain lust, among other things.
God has a far-reaching purpose for humanity, and his actions reflect that purpose, which will positively affect those who seek righteousness and will negatively affect those who seek wickedness. In other words, we will reap what we sow. People blame God when they don't see his purpose, but only their short-sighted desires.
This is the reason why God is the only source of objective morality.