Here's a list of gory/dark animes that I like: 

A/N: Not for kids^^ 

Hellsing Ultimate OVA

Hellsing, a secret organization of the British government, has long been battling supernatural threats to keep the people safe from creatures of the night. The current leader, Integra Wingates Hellsing controls her own personal army to eliminate the undead beings, but even her highly trained soldiers pale in comparison to her most trusted vampire exterminator, a man by the name of Alucard, who is actually a powerful vampire himself. 

*blood,explicit massacres,mutilated bodies,guns,swords,zombies,blood,blood,blood...

HIGH SCHOOL OF THE DEAD

In Japan, several high school students and a school nurse have banded together to escape Fujimi High School shortly after it was attacked by zombies. The group now attempts to figure who or what was responsible for this plague, and in the meantime, attempt to survive the present apocalypse.
*zombies,blood and LOTS OF FAN SERVICE

BTOOOM!

Ryouta Sakamoto suddenly finds himself transported from playing the hit Btoom! video game to being stranded on a mysterious island, equipped with a day's worth of provisions, a bag of bombs, a strange crystal embedded in his left hand and a huge gaping hole in his memory. Someone is attempting to recreate the ultra-violent Btoom! game in real life, and the island has been filled with an army of other unwilling players, each armed with one of the multiple variants of explosive weapons called BIM. Fortunately, Ryouta's an ace Btoom! player, but this insane version of the game has no reset switch or second lives, and there's only one way off the island: kill seven other people before they can kill you! Can Ryouta repurpose his game based skills fast enough to survive? 

*I had to skip the erted scenes. I found it so disturbing. /pukes

Higurashi no naku koro ni

After moving into the quiet town of Hinamizawa, Maebara Keiichi spends his days blissfully in school often playing games with his local friends. However, appearances can be deceiving. One fateful day, Keiichi stumbles upon news of a murder that had occurred in Hinamizawa. From this point on, horrific events unfold in front of Keiichi, as he soon learns his close friends may not be all that they seem.

*do not be fooled by innocent-looking girls
 

Umineko no naku koro ni

Umineko no Naku Koro ni takes place on the island of Rokkenjima, owned by the immensely wealthy Ushiromiya family. Because of the family head's poor health, this year involves the topic of the head of the family's inheritance and how it will be distributed.

However, the family is unaware that the distribution of his wealth is the least of Ushiromiya Kinzo's (family head) concerns for this year's family conference. After being told that his end was approaching by his longtime friend and physician, Kinzo is desperate to meet his life's true love one last time: the Golden Witch, Beatrice. Having immersed himself in black magic for many of the later years in his life, Kinzo instigates a ceremony to revive his beloved upon his family's arrival on Rokkenjima. Soon after, a violent typhoon traps the family on the island and a string of mysterious murders commence, forcing the eighteen people on the island to fight for their lives in a deadly struggle between fantasy and reality.

PUPA

The "life-and-death sibling" story follows Utsutsu and Yume Hasegawa, a boy and his little sister who find themselves all alone. One day, Yume sees a mysterious red butterfly and her body undergoes a strange metamorphosis—into a creature that eats humans. Utsutsu struggles to find a way to restore his sister.
 

MONSTER

*10/10! One of the most well-written mystery animes out there(no WTF? logic/characters here). It may be a bit long but it's totally worth it. 

Dr. Kenzo Tenma is a renowned young brain surgeon of Japanese descent working in Europe. Highly lauded by his peers as one of the great young minds that will revolutionize the field, he is blessed with a beautiful fiancé and is on the cusp of a high promotion in the hospital he works at. However, all of that is about to change with one critical decision that Dr. Tenma faces one night—whether to save the life of a young child or that of the town's mayor. Despite being pressured by his superiors to perform surgery on the mayor, his morals force him to perform the surgery on the young child, saving his life and forfeiting the mayor's. All of a sudden, Dr. Tenma's world is turned upside down by his decision leading to the loss of everything he previously had. A doctor is taught to believe that all life is equal; however, when a series of murders occur in the vicinity of Dr. Tenma, all of the evidence pointing to the young child who he saved, Tenma's beliefs are shaken.
 

info(c) wwww.myanimelist.net

MORE dark/gore manga & animes:

Elfen Lied
Genocyber
Urotsukidoji
Ichi the Killer 
Parasyte
Death Note \m/
Another
Shiki
Ghost Hunt

Ibitsu (Do you need a little sister?)

rabbit doubt 
Corpse party blood covered 
Deadman Wonderland 
7seeds 
Cradle of monsters 
Life Is Money 
Eden No Ori 
Doubt! 
Battle Royale 
Liar Game 
Dragon Head 
Cage Of Eden 
Drifting Classroms 
BM:Nectar 
Bokurano 
Psyren 
Aphorism

ENJOY! 

♥♥♥♥