The 80s had cool cartoons, and in 99,9% of cases they were about Good fighting Evil.
That said, very few of those shows actually had anything sinister; they tended to be campy.
It wasn't the case for the infamous INHUMANOIDS, a show so dark that it got cancelled after the first season - yet it still managed to make an impression and sell a few toys
Some sequences from the awesome and brooding Inhumanoids, where vast monsters lived below the ground and occasionally dismembered or transformed humans into something better.
"Inhumanoids; the evil that lies within.
From down in the fiery depths of the earth where nightmares begin"
You can suggest your own contender; it is futile though, cause Inhumanoids> other cartoons.
That said, very few of those shows actually had anything sinister; they tended to be campy.
It wasn't the case for the infamous INHUMANOIDS, a show so dark that it got cancelled after the first season - yet it still managed to make an impression and sell a few toys

Some sequences from the awesome and brooding Inhumanoids, where vast monsters lived below the ground and occasionally dismembered or transformed humans into something better.
"Inhumanoids; the evil that lies within.
From down in the fiery depths of the earth where nightmares begin"

You can suggest your own contender; it is futile though, cause Inhumanoids> other cartoons.