A just one. A non-arbitrary one. God.
He set the rules out to be obeyed and the penalty for breaking those rules.
He gave free will to choose.
We all broke the rules.
We turned our backs on God.
God justly judges us all.
But, by the same respect a merciful one. A loving one. A purposeful one. God.
He set the rules to be obeyed and provided a penalty for breaking the rules.
We broke the rules.
We turned our backs on God.
He changed the minds of those would would be saved and took their punishment for them.
Your qualm is that God isn't a universalist. You would rather him send all to Hell or take all to Heaven. Such is not justice. Such is not mercy. And such renders sin meaningless.
But some pay for it, some are rewarded. How do you get 'just' from this? It's purely arbitrary.