Cruel Intentions (dark manga)

A secretary whose overbearing love makes him manage every aspect of his young master's life is fearful to taint him. To vent his frustrations, he has rough sex with Miki, a free spirited but secretive man. However, as Miki becomes the personification of his object of desire, someone who he can unleash his twisted personality on, he begins to push and see how far their cruel play can go for.

rape, violent sex, manipulation, obsession, death
lies, two faced personality, illusion

Prequel to Chou to Hana no Kankeisei, this is the backstory to one of the main characters (which is only reference)
http://www.mangago.me/read-manga/chou_to_hana_no_kankeisei/

a child witnesses the sudden transformation of his guardian from a heartbroken man to a figure of obsession and possessiveness after the death of his mother.

rape, child

[collection]
1) a man falls for a colorblind butcher that slices his bulls and begins to stumble through life. 2) a man discovers a pair of women heels in his lover's closet. 3) a football fan become entangled with a security guard after a brawl at a game. 4) a young man types the words of an old author's story

animal cruelty (bull fighting)

I wouldn't say the stories featured are dark but they have a brutal overtone to its narration akin to stories typically read.

the guilt of pleasure when seeing the pain of someone close

guilt, mild violence

the lingering darkness underneath a perfect facade is showcased when a man's frustration is taken out on his young cousin.

rape, incest
two faced personality

the impure intentions of brothers who only have each other for support

the push and pull between brothers as one attempts to forcibly close the distance whilst the other loses his sense of self and freedom

rape, depression, blackmail

a world of flesh eating monsters and eroding sane minds