"unique part" is kind of a stretch there. The explanation for this used and abused character trait is so simple and obvious, it boggles the mind people don't think of it first. This is a shonen manga with a target audience still going to middleschool. That's why there is no actual depiction of cruelty in this world and that is actually the most fundamental issue with this story. That's why there is no inbetween, it always boils down to a "all is fine and dandy" or "Cataclismic death" situation. Doesn't let much space for nuance there, does it?
Rimuru being a happy-go-lucky dumb-dumb in an actual gritty world filled with monsters and somehow ruling over them, wouldn't be so bad. But that's clearly not what it is here. It really is just another typical isekai protag, it's the side characters that make it somewhat interesting, Rimuru by himself is just boring AF.
