I never contradicted you when you said that he makes the rules. But the rules are based on the nature of who he is and are not just arbitrary. He cannot change who He is - he is holy and holiness requires that he not be in the presence of sin. Sin is that which is contrary to his nature. he may make the rules to define the specifics but he cannot change his nature. And since He is the source of life those who are sinners must be out of his presence and must no longer benefit from his life-giving.
He is still merciful. The rules require that those who sin die. But he has provided a means by which all can avoid the just punishment.
For example,
Premise: God is the most perfect and good being in all the universe.
Premise: perfect and good beings should be loved
Conclusion: the rule is that you must love God or you are a sinner.
premise: I did not love God
Premise the rule is that one must love god or be a sinner
conclusion: I was a sinner
Premise : sinners cannot be in the life-giving presence of God
Premise : I was a sinner
conclusion: I must die
Premise: I must die
Premise: God arranged for me to not die
conclusion: God is merciful
