Demons aren't a part of nature though, everything that they are is unnatural and alien to meshi's world, they don't even need to eat.
IMO the lion definitely 'evil', at the very least because it has human or above level intelligence and is basically just making people suffer for his entertainment.
I wouldn't say it always looks ugly, I've seen some really nicely done manga coloring before, including panels from dungeon meshi, though there are definitely series that don't look good fan-colored, and from my experience most fully fan colored manga look bad.
Don't think its disrespectful to the artist though, basically any artist who has published work is going to see it edited/recoloured/redrawn, that's just how anything with a fanbase works.
No but I'm a drawfag. You obviously don't appreciate all the subtelties of Kui's art if you think you can just add or remove the coloring as you wish.
Just look at the way Kui handles it herself
: no greytone, no texture, no hatching because that would get in the way if the coloring.