I think it’s worse to be unjust.
Your question implies that even if I were merciless, I would be just. I’m fine with that. I don’t see a real problem with being merciless as long as you’re just. The problem with being merciless is that many times it involves being unjust (possible abuse, not granting mercy to the right person, etc).
Personally, I don’t have a lot of mercy in my life. I’m pretty chill and let things roll off my back but when someone does me wrong in a bigger way (cheating, badmouthing, betrayal etc), I tend to cut them out of my life and never let them back in. Ever.