Gibo

Gibo is an H-game and an anime with the title Stepmother's Sin, that has some dark themes, and puts the player in the shoes of a Villain Protagonist.

The game is kicked off by a flashback where your protagonist (Yusuke Yagami) remembers watching his mother having sex with a man not his father, and since then assumes all mothers are secretly cheating sluts who deserve whatever suffering they might get.

That said, his father remarries to a woman named Misako, and we actually get to see what would happen if he fufilled his dark desires in this regard. Throw a naive new stepsister (Shiina) into the mix, and this is a really dark game.

H-Game

 * Asshole Victim: Yusuke's default view of Misako, albeit minus any potential sympathy.
 * Shiina also gets hit with this, albeit the lion's share of his hatred is directed at Misako.
 * Beauty Mark: As seen above, Misako has one.
 * Big Damn Villains: Yusuke can technically get a moment or two of this in some scenarios, though it's really hard to cheer for him.
 * Complete Monster: Yusuke has no problem crossing the Moral Event Horizon if it means exacting his revenge on unfaithful women.
 * Downer Ending: Technically all of them are this. Either you die, or Yusuke, who is a total Jerkass at best, gets away with sexual torture and extortion scot free, and gets to keep getting laid with one [or more] of the other protagonists to boot.
 * Expy: Yusuke Yagami looks like Light Yagami somewhat. It doesn't help he has a cynical, nihilistic frame of mind, and that he has already decided the targets of his hatred [cheating mothers] will be punished by any means necessary.
 * Freudian Excuse: A somewhat petty yet plausible one for your protagonist.
 * Hanging Judge: Yusuke already declared Misako guilty of being a cheating slut before that was even proven, and was already planning to punish her.
 * Hot Mom: Misako... though that is not a fact to her advantage. Yusuke's actual mother, seen in flashback in flagrante delicto, was also this.
 * H-Game POV Character: Yusuke is a Type VI A.
 * Kissing Cousins: More like have had sex repeatedly and shamelessly cousins.
 * Moral Event Horizon: Yusuke believes his own mother crossed it by screwing another guy, and in turn he crosses it with his sadistic desire to make every unfaithful slut who crosses his path suffer.
 * Incest Is Relative: Your protagonist's occasional lover is his cousin (Mio), who is basically his female counterpart. In fact, EVERY possible coupling in the game falls under this.
 * If you wind up with your cousin, it also overlaps with Unholy Matrimony.
 * Jerkass Woobie: Your protagonist is supposed to be this, but he comes off almost entirely as a Complete Monster instead. Mio can come off as this in at least one ending.
 * Love Redeems: In the literally-named "Redemption" ending, Misako and Shiina run away, but Misako returns and reveals everything to Yusuke. They manage to forgive each other, and then live together as a couple (despite now knowing they actually parent-and-child), as they are to become parents together of their own child.
 * Manipulative Bastard: Yusuke. Mio has her moments too.
 * Magnificent Bastard: In some endings, you just have to admire how well things have worked out for him, even if he is the world's most loathsome prick.
 * Moral Event Horizon: This is actually measured as a statistic in game (the Lust Meter), with the worst endings occurring the higher it gets. The other meter (the Love Meter) is technically the Pet the Dog meter.
 * NaïveNewcomer - Misako and Shiina.
 * Neutral Evil: Yusuke by default (he publicly comes off as Chaotic Neutral), though you can go all the way to Chaotic Evil if you pick the most sadistic choices.
 * Pet the Dog: Despite being a total bastard, you are given the option to have your Villain Protagonist do something somewhat noble on occasion.
 * Sick and Wrong: Yusuke is willing to do some very depraved things in his quest to make "cheating whore wives" suffer.
 * Son of a Whore: Why Yusuke hates his mother.
 * Stepford Smiler: The protagonist, played totally straight.
 * Too Dumb to Live: Yusuke sees the targets of his rage as this, though in some of the bad endings this comes back to lethally bite him on the ass.
 * Villainous Incest: All possible sex partners for Yusuke are related to him. Taken even further with his relationship with Mio, who's not exactly a nice person either.
 * Villain Protagonist: It's a game mostly about the protagonist raping people.
 * What the Hell, Player?: Some choices, such as being too perverted or outright raping Shiina when Misako has begged you to spare her, prompt a bad ending in which Misako leaves the household and leaves a letter berating Yusuke (and, by proxy, the player) for his actions. Some of them are mildly humorous ("You pervert;") but most are outright WTHP?s.
 * What the Hell, Player?: Some choices, such as being too perverted or outright raping Shiina when Misako has begged you to spare her, prompt a bad ending in which Misako leaves the household and leaves a letter berating Yusuke (and, by proxy, the player) for his actions. Some of them are mildly humorous ("You pervert;") but most are outright WTHP?s.

Anime

 * Gainax Ending: Near the end of the hentai anime, we see Yusuke being brought home by the police only to see Misako and the three guys she previously had sex with. Suddenly, a gun goes off, leaving questions as to whether Yusuke shot his mother or himself.
 * Hey, It's That Voice!: You'll never hear Yami "Yugi" Moto the same way again after hearing Dan Green as Yusuke.