Thats definitely http://www.mangago.me/read-manga/zankoku_na_kami_ga_shihai_suru/
Mangago genre tags are not always right. So it's safe to always re-check with baka updates. It's one of My Fav manga. And trust me, it's definitely NOT yaoi. The ending is not exactly tragic but not exactly happy either. It's bittersweet & kind of open ended as the mangaka leaves plenty hints for the audiences to connect.
You should definitely finish it. What chapter are you on right now? Bcz it only gets better and better. It has a very realistic ending. Not like those typical mangas that forces the plot to have a happy or sad ending. That manga is very realistic, and happy endings are too good for reality. So they end it in a very subtle way. Not a sad nor happy ending. I'm really satisfied with the way it ends. Fucken beautiful.

It's about a 15 years old boy who got a single mother (bcz his biological father died). Then her mother remarries a guy, then this guy became this boy's step father. But this boy's step father is actually a gay sadistic paedophile. His step father blackmail this boy to have sex with him, or the step father will dump his mother. His mother is a very suicidal person, so the boy gave in. Then this boy got sexually abused repeatedly, until the boy got mentally unstable. This boy had enough, and he plans to kill his step father. Anyone know? I've read this long ago, but I completely forgot the title! The manga was made in the 90s, a pretty old manga. really disturbing content, but very well written.