If someone were doing something harmful to themselves, would it be merciful or just to stop them? Say, for instance, that you are an engineer, and a neighbor was building a structurally unsound addition to their house, which would likely result in the addition collapsing, potentially injuring someone. And despite your best efforts, they will not listen to your professional counsel. Would it be more merciful or more just to call the city officials, urging them to make an inspection of your neighbor’s handiwork?
On the one hand, it would be just. You would be taking the relevant laws and rules into account, holding them accountable in the process, in such a way as to make them liable to different fines and consequences. They may feel that you overemphasized justice and rule keeping, feeling angry with you for interfering in their private affairs. On the other hand, it would be merciful, for what could be kinder than to do what is necessary to protect the family and possessions of your neighbor from great danger, even in the face of their opposition?
In this case, Plato would say, it would not be merciful to leave them to their own devices, risking their lives and that of their family and guests, and neither would it be just. The only way to truly be merciful is to pursue justice, for only justice will bring about a situation which is good and safe for everyone involved.
Adam Johnson on why pursuing justice—say, through pressing one’s own case against a sexual harasser—may in fact be true mercy in the way of the cross.