Mercy, in my view, can only be virtuous under conditions where justice is offered.
Essentially, where a wronged person forgoes legal action against the one who has wronged them.
I don't consider it merciful to pardon somebody who has wronged someone else, or to pardon somebody who has wronged nobody (to pardon them would be justice, not mercy).
As such I consider justice more important, though I realise my company in this endeavour will likely be a bunch of malicious blowharRAB who want to see their enemies suffer.