Dark Graphic Novels and Manga?

Fun Home by Alison Bechdel can be summarized by this line from its stage adaption: “My father and I grew up in the same Pennsylvania small town, and he was gay and I was gay, and he killed himself, and I became a lesbian cartoonist.” It’s a memoir that deals very heavily with themes of childhood abuse, homophobia and of course suicide. I don’t know if it’s dark in the way you’re asking for because it’s not exactly action-packed, but it’s a rough read.

Watchmen