It might depend on how much being merciless involves being unjust.

• As a gut reaction, being brutally merciless to other people seems somewhat, I think, worse than being unjust, maybe since I associate merciless actions and behaviors with more violence than I do unjustness.

• However, I think it ultimately depends, because oftentimes being merciless involves being unjust - showing no mercy to someone who didn't "deserve" it for doing something bad, possible forms of abuse, etc. If being merciless has a measure of "justness" to it (because you're showing no mercy for someone who has done something terrible), then it might be worse to be unjust, because with unjustness you are by definition treating someone badly who didn't "deserve it" in any way, necessarily.