I agree God doesn’t sin. Whatever God does is true and just.
Our understanding of what’s true and just, however, could be much different than Gods.
As the heavens are higher than the earth,
so are my ways higher than your ways
and my thoughts than your thoughts.
For example: God can takeaway life, without being sinful because all lives comes from Him and therefore belong to Him. Whereas it is sinful for humans to take other people’s life because we do not own others life.
However, a theological definition that says “God cannot do something against His nature” is inconsistent to the fact that if there is anything a god incapable of, then he is not god.
The question is whether we can define “God’s nature” accurately. And then, if we can define god’s nature accurately, then the next question whether that definition actually limit God who supposedly beyond our capability to define Him. So, consequently that definition most probably is imprecise if not completely errs.
Thomas Aquinas philpsophy is very brilliant. It has it’s place in academy, education, and so on. I am afraid it is not enough to define God. We should trust Jesus words more than Thomas Aquinas philosophy. His philosophy I am sure has limit, while God is without limit