Koroshiya 1, Homunculus Recommendations
Crude and rough drama that does not mind becoming violent or explicit, but is using said explicity not mainly for shock value, but as a tool to show human psyche. Follows cast of various characters which some of them (most) might have a screw loose, including the protagonist. Both manga are also beautifully drawn, and as they are made by the same author they also have similar artstyle and feel.
I feel like Homunculus is another great piece of work by Hideo Yamamato, considering if you really liked Ichi the killer in fact. Some people might find it a bit weird but it doesn't matter what people think in general. There was some cool parts to it and i read it in 2 days. 15 volumes seems like a lot but if you start you'll finish just as fast