I have a fondness for flawed and/or tormented baddies, but when you're talking about truly dark and twisted villains, you need baddies that LOVE being bad. Demons should have a passion for their work of sowing death, destruction and corruption wherever they go.
Demons mean collateral damage--nothing should be safe when one is around. Target beloved NPCs or wipe out whole communities; spread blight, disease or madness; inflict lasting damage or effects that can't be healed with a simple spell or a couple of nights of rest; and have the demon do it all with a smile on its bloody maw of a face.
Basically, just pile on the stakes making it imperative that the heroes defeat the demon--and then make it a bloody, brutal fight to remember with the demon fighting to the last with every nasty trick it can think of.
I have a fondness for flawed and/or tormented baddies, but when you're talking about truly dark and twisted villains, you need baddies that LOVE being bad. Demons should have a passion for their work of sowing death, destruction and corruption wherever they go.
Before you get too far into this thread, I think you'd do well to actually define 'dark'. If you could actually define 'dark', you might end up with some answers to the questions that wouldn't be just restating pop culture demonic tropes.
I thought I did that. Albeit a bit tongue in cheek.
Dark is the absence or scarcity of light![]()
I do think you have a good point, but I was stuck in smart alec mode.
I suspect dark is related to scary, but not entirely the same thing. Much as dark humor is funny, but not entirely the same as funny.
Since I'm feeling a bit guilty by the fact I'm being unhelpful, I'll go ahead and at least answer my own challenge: dark is the theme that evil is in control over the world and will triumph over good. That is to say, dark is a metaphor for evil and light for good, so if you want something to be darker what you do is deny the presence or power of good.
Not to get actually political here, just showing how far the Demonic Corporate Conspiracy could be taken.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.