The phrase "a much more sinister" is correct and usable in written English.
It can be used to describe something that is significantly more evil or threatening than something else.
Example: "The new evidence revealed a much more sinister motive behind the crime than anyone had initially suspected."
Alternatives: "a far more malevolent" or "a considerably darker".
But I think it can be shown to make a much more sinister appeal.
Father Leon would have made a much more sinister dream villain, with his thick accent and his formal English.
I wanted that version of Robert to exist alongside the possibility of a much more sinister one.
Luke had a much more sinister view about what had happened to Bluie less "Vertigo" and more "Psycho".
"What would people say?" They worried, as if these shocking details were just the first hints of a much more sinister biography.
For example, the "Thin Man," who in the standard version appears to be a glorified butler to the city's all-powerful founder, turns out instead to be a much more sinister figure, a combination of spy and detective.
It would risk revealing the aces up Monahan's sleeve's to divulge the exact nature of their conversation and the ramifications that grow from it – suffice to say that car theft, a missing person and a subject much more sinister are canvassed.
In truth, Mr. Buffett said, a specter much more sinister than corporate taxes is looming over American businesses: health care costs.
A seemingly much more sinister group of plants, the holoparasites, get all their nutrition from their host plant, producing no chlorophyll themselves.
Something pernicious has happened in the 15 years since, and Sydney has not just regressed into a ghost town, but there is an undercurrent of something much more sinister in the way the city is being run.
So she sets off with the Swallow Man on a never ending adventure away from the crippling dangers of Krakow, toward a life filled with much more sinister threats.
