@TrueGoddessReincarnation The "Merciless" skill isn't one that can just be acquired like that by just being merciless.
All skills have lots of conditions to acquire them normally, and unique skills are most of the time actually unique. It's possible for there having been another "Merciless" skill in the past for someone else, but it likely had a slightly different effect.
According to Veldora, Unique Skills are representations of one's ego(the personality part attached with memories to the soul). Due to that, it's normally very rare for someone to have more than 1 Unique Skill (and already having a single Unique Skill is rare since it's not just a representation of ego as a skill, for it also has conditions that have to be fulfilled to acquire such a skill).
My hypothesis on one of this "Merciless" Unique skill is that the holder needs to already have a Demon Lord Seed and/or have access to a way to interact with souls.
Another reason why Rimuru got such a skill would likely be due to how drastic the change in his viewpoint of things became. He's not just someone who is merciless, he's a very compassionate person who suddenly had a drastic change in his modus operandi. So the drastic change in an aspect of his psyche led to him gaining this "Merciless" skill.
So, all in all, the two reasons I mentioned above along with Great Sage's possible intervention in this skill acquisition (so to facilitate Rimuru's task of becoming a Demon Lord) are likely why he gained such a skill at such an opportune moment in such circumstances.