Depressing Books/Mangas

I'd check out pretty much any of Cormac McCarthy's work if you haven't already. Namely Blood Meridian, The Road, and No Country for Old Men. Those are his best and darkest works (imo)

I dont know if im replying to your comment here but anyways...
I've read Blood Meridian by Cormac and watched the movie No Country for Old Men, but hes more disturbing than depressing imo, i mean his works are excellent

Villains Are Destined To Die by Gwon Gyeoeul. Both the comic and novel are so good

Mangas:

Fire Punch

All you need is kill

Himitsu 

Genderswapped Medic’s battlefield diary

What do you wish for with those murky eyes

Given

Weathering with you

Manwhas:

Mission: Save the Hunter 

The Horizon 

Novels:

Goodnight Mr.Tom by Michelle Magorian

Smart by Kim Slater( I read this ages ago but the ending really got me as a kid)

Hiii I'd really recommend Aku no Hana, It's a really underated manga and it's not really that long