Not horror, but dark. Two I just streamed on Netflix and enjoyed quite a bit.
Sightseers - British black comedy, I guess. Much more black than comedy, though. Natural Born Killers-type couple on a murderous vacation. Both deranged oddballs. 7/10
Snowtown Murders - Extremely dark and disturbing biopic of Australia's most notorious serial killer. Some super tense scenes involving a few severely damaged mutherfuckers. Very well done. 8/10