This is going to be pretty vague, but I hope you'll be able to find something.

I'm looking for a dark manga, either fantasy, or sci-fi, about a motley crew of characters on a personal mission. Characters who are absolutely irredeemable scum of the earth - sociopathic murderers, rapists, folks with all sorts of... less pleasant types of -philia etc., basically the worst of the worst, who fight neither for good nor evil, but purely for themselves and their own personal gain. Characters who are determined to accomplish their goals and feed their metaphorical inner demons to the point of being utterly ruthless, capable of doing anything that brings them closer to success, even if it means commiting unspeakable cruelties towards other beings, be it human or not.

Can you think of anything that fits this description? Finished manga only. Thanks.

Koroshiya Ichi fits pretty well, I think.

Also, many of the titles by Matsumoto Jiro, such as Freesia and Netsutai no Citron.

As for ongoing series (in case you want to keep an eye on them) : Sunabozu, Overlord.

Quote from calstine

Koroshiya Ichi fits pretty well, I think.

I've already read it, but yeah, character-wise I'd say it's pretty much what I'm looking for.

Quote from calstine

Also, many of the titles by Matsumoto Jiro, such as Freesia and Netsutai no Citron

I checked the list of his works out. Most of them aren't translated, unfortunately, and the ones that are don't really strike me as something I'd wanna read at the moment (mostly because I'm looking for a more conventional fantasy/sci-fi setting). Thanks anyway, though.