There are various Manga as well as Anime that questions the sanity of a creator. Here are some of the dark manga as well as anime:
1.Berserk- Kentaro Miura
It is a dark historical manga which also includes some fantasy elements in it. This manga is going on for more than 20 years. It have a loyal fan base. This manga speaks for itself that why it is so good. The story is about a man which loses everything in his life. He had a very dark childhood like the friends which were near to him betrayed him as well as family members were tortured in the worst way possible. It is one of the best dark manga out there and it is 18+.
2. Elfen Lied
Well, having superpowers is one of the best thing in one's life or not? think again after watching this anime. In this anime our main character is a girl who gets superpowers one day but due to this incident she is locked by the government in some private facility and lots of experiments are being conducted on her. She bears unimaginable pain in that facility just because government thinks she is dangerous for the society and they want to misuse her power for there own. Well, during one incident she gets away from that facility and got spilt personality issues where on one side she is having a very nice behaviour and on the other side a terrible nature. It is certainly one of the darkest anime to watch.
3. The Future Diary
This is one of the best anime to watch right now but is quite dark. In this anime there happens an interesting thing that one day one candidate is choose to be the God who will work as a God before he dies. Well, the cost of being a God is not very cheap it comes with a high price too. In order to maintain being God the candidate will have to kill the other candidates and be the last one standing in the end. There is a shocking turn in the story when a girls suddenly confess her love for that candidate and doesn't allow any other person to come close to him and will immediately kill him. She is a complete psychopath who is in love with him. Well, we found out that she is the main protagonist of this story and she continuously saves him whenever he is in any kind of danger. It is one kind of its own anime.
4. Tokyo Ghoul
It is a pretty interesting anime in which a common boy goes on his first day which is a very special day of him and this is the day which he will never forget. After going on his date he realizes that he is a semi-ghoul. He founds out that he is a mix between a human and a ghoul. The story gets pretty intense as it moves forward as the main character have to cope up with his eating problem as he can only eat human flesh and whenever he sees a human and he is hungry his urge to eat human grows even stronger. But in the positive note the main character super human like powers but also on the same side he have to daily with other ghouls as well as other humans too. Alongside this he have to have many deaths which comes side by side. Also he have to daily see Trauma, torture which follows with these powers that he got and also is the downside of getting these powers. This is a pretty interesting to watch.
5. Battle Royale
This is one of the best dark manga out there. The story of this manga takes place in a fiction Japanese place in past. In this story government controls everything after a war which happened recently. After the war government started a military program and the program's main aim is kill or be killed. In this randomly students are selected from high school in either way being it kidnapping, dropped trip etc. The students are places in an island where they are forced to kill each other in order to survive and are given weapons, survival kits and a collar is placed around their neck. The students then desperately fight each other in order to win and if not they will be automatically killed if they don't kill any student under 24-hrs. It can said to be one of the darkest manga that i have ever read and it questions the sanity of the creator.
It is also dark fantasy type manga which is very similar to Berserk manga. But the twist is that due to a plague named Yoma there are beings called humanoid shape shifters that consume humans. Well, it is a very dark manga that is hard to digest. Claymores are the female warriors who fight against Yoma. Just like Yoma Claymore also have superhuman humanities which they use to fight against each other. The island is divided into 46 districts and each district is assigned one claymore who is classified according to their abilities and placed according to that.
These claymores possess their own unique sword style which they transformed with the help of their Yoko abilities. Yoki is the demonic entity who granted powers to both Yoma and Claymores. It is definitely a very different kind of manga, the story is unique but the content is quite dark. So. watch if you are able to digest such content which involves lot of blood.
The main character of this manga are Kirie Goshima and Saito. They live in a small town named korouzo and one day something strange events started to occur. These events occur on a regular basis in a spiral form. From that day onwards people of that town face a change in their behaviour as they began to take interest in those spirals like incidents and became fascinated by them which may be destructive. Shuichi wants to leave as soon as possible from that town but the spirals don't seem to let them go.
8. Petshop of Horrors
The main character of this manga is Count D who owns a pet shop which contains a lot of creatures in it. The creatures which are inside that shop are also not normal they are supernatural in nature. The customers can take these creatures with them only if they meet certain contract and if they break that contract they have to face consequences. This is is very dark manga as Count D is very dark in nature and people after leaving Count D pet shop seems to lose their mind.
The main character is Kaiman who got transformed into a lizard head with the ise of magic. Kaiman is on a mission to search his real identity and during which he tortures many wizards on the way whom he things have the answers of his questions. The En family is a powerful family which sends troops of wizards to kill Kaiman after they notice his deeds. It slowly transforms the world they are living in.
10. Pandora Hearts
There is a story about monsters known as chains living in abyss in order to scare small children. Oz Vessalius is the main character of this manga and is also heir to the Vessalius family. They don't believe in Chain but the story takes turn when during the coming of age ceremony Oz is charged in abyss and he forms contract with a chain who takes human form. If is a pretty dark manga.1.