A balancing act
"Children are innocent and love justice," Chesterton wrote, "while most of us are wicked and naturally prefer mercy."
I'd amend that to, children love justice for others, and most of us adults prefer mercy for ourselves. Few children are overly keen on dispassionate justice for themselves, and plenty of sufficiently wicked adults object to mercy for others.
Children and adults: not so different.
It takes some maturity, I think, to see the relationship between the need for mercy for oneself and the desire for mercy for others. Heck, it takes some maturity to see the need for mercy for oneself in any but a superficial sense.
Once we properly desire mercy for others, the justice we desire for them is not merely punitive, but restorative and even healing. Someone at that level of maturity should then see the need to endure healing justice himself (and please let me know when you get there if that "should" holds).
Extending the circle of concern.
(This post uses "mercy" and "justice" from the perspective of the wrongdoer. We can also speak of mercy toward and justice for the innocent, whether us or others, but somehow that's an easier balance to reach.)