That's a very interesting question. I think on balance that mercy is more important than justice. There's a line in Hamlet:
" use every man
After his desert, and who should 'scape whipping?
Use them after your own honour and dignity: the less
They deserve, the more merit is in your bounty."
I don't think it would be a better world if everyone got exactly what they deserved all the time. For example, I'm opposed to the death penalty. Someone who commits murder probably deserves to die but that doesn't mean you have to kill them. Or, as a less extreme example, if someone slaps you, you can always turn the other cheek. Justice is satisfying, but often forgiveness is better.
Ruthlessly Unforgiving is the worst of the two in my opinion.
It brings up examples of 'families' of parents-children-brothers-sisters, you name it.
What about friends,neighbors, and of course fellow man.
I don't see why it's an either/or question. I think being ruthlessly unforgiving IS being unjust.
However, I think justice is a very important virtue, so if someone was knowingly and deliberately unjust, that would be very wrong in my opinion. I think we need to strive for both justice and forgiveness, when the boot is on the other foot, we need people to give both of those things to us too.
If the punishment does fit the crime then it would be going easy on the culprit not to carry it through. Think of all these murderers who get minuscule sentences. That's not right. How is justice being served if criminal is given the mercy of a short sentence as opposed to life, etc?