Can anyone give me some manga recommendations that are like really good and pretty dark? S...

This is the first manga I thought of when you said something really deep and pretty dark. Enjoy!
http://www.mangago.me/read-manga/homunculus/

If you don't mind the lenght:
1) http://www.mangago.me/read-manga/zankoku_na_kami_ga_shihai_suru/
2) http://www.mangago.me/read-manga/let_dai/
3) http://www.mangago.me/read-manga/j_no_subete/

And a little bit shorter:
http://www.mangago.me/read-manga/smells_like_green_spirit/

+1 on Homunculus...my absolute fav psychological manga

This list may help you: http://www.mangago.me/home/mangalist/235645/