You mean how to argue that an eternal God can only be good not evil? Satan (evil) can be contingent while God (good) cannot?
You would have to argue that evil depends on good. If evil is contingent on good, then God can only be good, because good can exist by itself (like God) while evil (Satan) cannot. Which is why Satan is not equal to God.
We have no experience of someone being evil for the sake of being evil. The closest we can get is cruelty, but people are only cruel to pursue something good, like power, safety or pleasure. Another example is adultery, people commit adultery to seek love or connection with someone - which are normally good things.
In contrast, you can do a good act for the sake of goodness. E.g. you can feed a poor person simply because you feel compassion and want them to feel better. People can, and do do kind acts even if it offers them nothing in return, but because it is the right thing to do.
Evil is nothing more than the persuit of something good in the wrong way. I.e. the perversion of good.
I hope that was your question and I answered it.
Here are the full premises for you to have a look at
P1: It is logically impossible for someone to be evil just for the sake of being evil
P2: if it’s logically impossible for someone to be evil just for the sake of being evil then it is logically impossible for evil to exist on its own
C1: Evil cannot exist eternally on its own
P3: if it’s logically impossible for evil to exist on its own then evil is a perversion of good
P4: If evil is a perversion of Good then only Good can exist on its own
C2: Only Good can exist eternally on its own
P5: God is changeless and eternal
P6: Since God is changeless and eternal then an evil God is impossible since evil cannot exist eternally on its own and God can’t change from Good to evil since he is changeless
Final Conclusion: Therefor it is logically impossible for an evil God to exist and it’s necessary God is good.
