Like phatic says, it all depends on what kind of music you're looking for. There is "dark disturbing" music in every period (Baroque, Classical, Romantic, etc). But I'm a specialist listener in the Romantic period piano niche, so I think I have to vouch for Charles Valentin Alkan as a composer of dark, depressing, agonizing music from the abyss. However, you may be looking for atonal stuff and so Alkan wouldn't qualify.
In any case, I must recommend the L'enfer or "Hell" movement from Alkan's Grand Duo concertant in F sharp minor Op. 21 for piano, violin and cello. I don't know what Alkan was thinking when he wrote it, whether the pure evil piano chords mean to depict Hell as a physical place, like a dark void, or whether the mourning violin is referring to Hell as a state of mind, like hopelessness and regret. It might not be your cup of tea if you're into Shostakovich string quartets, but in my opinion, the L'enfer is a masterpiece; it's one of the best musical manifestations of Hell I've ever heard. Unlike Liszt's Dante Sonata/Symphony, Meyerbeer's Robert le Diable, or Mussorgsky's Night on Bare Mountain, it doesn't wail and crash with infernal sounds or scherzo-ish demonic noises... it instead focuses on the sickingly disturbing "pain" of being in Hell. I really think it's a grand composition: bleak, slow, dissonant (not atonal) and a real musical excavation into the spiritual suffering one could experience in Hell.
Overall, it's a "disturbing" and yet lyrical work that I could recommend to anyone for a taste in Romantic period chamber music. But hey, this is just my opinion
