Grace, by definition, is unjust. It is not giving us what we deserve, but giving us the opposite. It is why just grace is an oxymoron. If God puts his grace upon us justly then he is giving us what we deserve. But we do not deserve God’s good favour, that is what grace is!
I wonder if you have ever thought about the opposite of grace? We all know (I suspect) that grace is unmerited favour despite what we deserve. It is more than just unmerited favour because you can put your favour on someone who hasn’t done anything warranting your ire. Grace is unmerited favour in the face of what we deserve. God shows his grace towards us by showing us unmerited favour in the face of the wrath and judgement we deserve by nature.
What, then, is the opposite of grace? Some would argue it is judgement. After all, if we don’t have God’s grace on us, we stand under his wrath. We will face his condemnation. But that is really the result of not receiving God’s grace. Or, more accurately, the result of our own sin. It isn’t the opposite of grace, just what results if we don’t receive God’s grace.
Look again at our definition above. God’s grace is his unmerited favour in the face of what we actually deserve. If we do not have God’s unmerited favour in the face of what we deserve, we must have God’s wrath in line with what we do deserve. Grace is undeserved so what happens apart from grace is entirely and properly deserved.