Which episode is that? Berserk is a bit dark, otherwise I don't think there are that many dark ones that I have watched ? Is attack on titan dark? Vinland Saga? Gantz (Think it only exists in manga form) ?
Just remembered some that I've accidentally watched and/or forgotten: Blood-C (don't be fooled w/ it's bright coloring & CLAMP's cute drawing style, it's bloody as hell for sure) Witch Hunter Robin Ghost Hunt (those who remember watching the early seasons of X-Files from the '90s w/ Fox & Dana, would probably find this to have +/- similar vibes) Blood: The Last Vampire These are more of dark comedy: Black Lagoon 1st season, Drifters & Hellsing Read or Dream Tv series (continuation of Read or Die OVA, it's the plot twist from the first that turn the Tv series into a darker one IMHO) X/1999
its an old title, but Now and Then, Here and There had some really heavy and dark themes for the, then, 12 year old me.
I don't consider death note as a dark anime, so i won't suggest that imo is just a high-level iq type detective plot, where mc went wrong or psycho in a way. That's it. Egoist, God complex....etc. It is not a story where i will feel dark. Same with youjo Senki, So they are just fun anime in my list. So i excluded such animes,etc. Monster ( old anime, but it has a good story that gets darker, Psychological Genre ) Another Attack on Titan ( my favorite for now ) Blue Gender Talentless Nana Happy Sugar Life Elfen Lied ( a little boring at times, but good time pass ) Texhnolyze Future Diary, mirai nikki ( a good story ) Higurashi: When They Cry Neon Genesis Evangelion ( bad animation though ) Made In Abyss ( though the style wasn't that good ) Paranoia Agent ( bad art, considering the time i am recommending lmao ) Puella Magi Madoka Magica