Suffering without purpose is, to me, bad writing. You never want to be a sadist to a character, you want to give a character life. Life is not roses, nor is it sunshine and farts, nor is it mere survival. It is a terrible cocktail of all that, thorns included, and you'll like a bit of the taste but probably not the whole beverage. A dark character whose parents are dead, has been raped, has been prostituted, has killed at an early age could be compelling; though rarely is that the case. Often it is edge for the sake of edge (which can have a place too).
But it's a matter of perspective. A child hero or a child soldier? The ramifications of taking a life at a young age, or being subjected to ones first real bout of human cruelty. The rocking of the boat until it tips over or your fellow sailor beats you to death with the oar.
A dark character to me, shouldn't be put through Hell. They should walk into it obliviously, while doing everything in their power to avoid entering Hell. They don't see the Hellmouth until they've toppled down into the abyss. And it's the death by little cuts of if they can come back from it. The sacrifices that build up, the little death by inches of losing who you were to be who you are and fearing that who you are might not be who you'd like to be. Or even who you have to be to survive this.
Suffering has to be a human endeavor to taste sweetly. The hand of the writer has overcome the hand of God when it comes to hubris, and this might be due to how our world often has the bad guys win. Nobody knocks them down, but themselves, and the damage is done.
Sorrow is something you have to feel deeply. It might not ever drive your character to cry, but it is why under their skin they're screaming. Anger can and often should be inward; the character only hits his friend because he can't undo something interior and the frustration manifests and that cancer of anger grows. Loss should always have ramifications.
The man on the pile of corpses is the villain, the legend; his victims are a statistic. The corpse of a close friend and the fact that nobody ever found the killer is the death of trust and the ever present elephant in the room. A man who chokes someone out is scarier than a man who spirit bombs a crowd. The man who chokes someone to death could get close enough to do it.
Suffering without that bit of human integrity is sadism for the sake of sadism. We've all been there at various stages, nor am I accusing you of this. I'm sort of talking in general on the subject. A hero who fails at their goal has wasted countless pages; a hero who succeeds but loses something to their integrity makes the victory bittersweet; and that's real life.
You can pay off your student debts, but that 10k is gone from your bank for something rarely worth the amount. You're free, but it is bittersweet when you see that 10k is a lot to spend but little to have.
I like a good dark story, but rarely are they dark for me. I've been through the wringer; who hasn't nowadays? I think I get more disappointed when I see something that could appeal to me in theory, die upon execution. I remember a gay little comic about two serial killers in NYC. They had to be cute. They had to be funny. There was no depth. Killing was just a quirk, not the hollowness and lack of human empathy coming to boil over. I remember seeing it and thinking "Oh the art is nice, but these characters belong in an anime not hacking people apart." I forgot the name of it; perhaps it has improved.
I like to think I'm telling something of a human story (really one about family and death by inches), and calling it dark has that unfortunate tag of being edgy nowadays due to an oversaturation of "I was raped/orphaned/hurt and now I don't feel anymore" stories. I'd love to see more proper, dark work; but that takes time and twists and turns.
Bone, for example, is an amazingly dark comic once the Lord of Locusts puts its plan into action. There are haunting scenes. I wouldn't label it dark by today's standards. Though perhaps I'm just burnt out on labels.
Give my comic, The Sisters a shot if you'd like. Things pick up plot-wise during the Long Night; and it's been a ride ever since. Whole point of the antagonist is sort of a death by cuts nasty, even if it won't be clear til next week. He's a case of spoiled integrity, someone who has had to do things to survive and now he has to keep doing things to slither through life before fear catches up to him. The main characters are all broken to some degree, though it is only openly spelled out for the middle child; the youngest is starting to catch on later and the eldest is clearly an alcoholic without having to go full drunk for the sake of spelling it out.
I'll check out your work when I'm not at a NJ truck stop. It's interesting to talk about these sorts of things. And while these are my thoughts, by no means are the mono-dominant. Just my perspective having worked on dark stuff before, been through dark stuff before, and working on something in the field at the present.