Sasuke could also be seen as a 'Psychopath Killer' in certain fan - made stories. After he left the village and his thirst for revenge grew, he became more and more ruthless. He might be written as a character who loses all sense of morality and goes on a killing spree in some darker fanfictions.
One interesting 'Naruto Psychopath Killer' fanfiction could be where a new villain emerges in the Naruto world who has psychopathic tendencies and targets the Hidden Leaf. This villain might use unique and cruel killing methods that no one has seen before.
Sasuke could be a potential rival. In many Naruto fanfictions, Sasuke is highly skilled in ninjutsu and sharingan - based genjutsu. His rivalry with Naruto, if Naruto becomes a genjutsu master, would be intense. Another could be Itachi. His mastery of genjutsu, like Tsukuyomi, would make him a formidable opponent for Naruto in the genjutsu realm.
Even a new character created specifically for the fanfiction could be the serial killer. This character could be someone with a grudge against Beacon Academy, for instance. They might be killing students or professors in a way that ties into some dark secret or vendetta they have against the institution.
The Hidden Villages could be antagonists. If Naruto turns into an insane killer, the villages would see him as a threat and try to stop him. They might send their best ninjas, like Kakashi or Might Guy, after him.
Well, perhaps the bloodline could grant superhuman strength like Killer Croc has in the DC universe, which in the Naruto world might translate to being able to break through earth - style jutsu defenses more easily. Also, it could give the ability to swim at great speeds and hold one's breath for long periods, useful for underwater missions or battles near water sources in the Naruto world.
One potential enemy could be Danzo. In the original series, Danzo had many shady dealings. If Naruto turns cold - hearted, he might see Danzo as a threat to Konoha's true peace and go after him. Another enemy could be Kabuto. After his experiments and all the chaos he caused, a cold - hearted Naruto might not tolerate his existence. Also, there are the Akatsuki members. Naruto could target them in a more ruthless way, especially Pain, who once attacked Konoha and caused a great deal of destruction.
Hinata could be a great choice. In the original series, she has always had feelings for Naruto. In a fanfiction, they could finally get together more easily. Maybe after the war, Naruto realizes how much Hinata means to him and they start a relationship.
There could be an entirely new character created as a necromancer in Naruto fanfiction. This character could be from a hidden clan that has long been rumored to have necromantic abilities. They could enter the Naruto world and disrupt the balance between life and death, causing all sorts of chaos and new storylines to unfold.
One possible plot could be that Naruto is exposed to a very dark and powerful jutsu that corrupts his mind. This jutsu could be some ancient, forbidden technique that no one was aware of its full power. As it takes over his mind, Naruto's values change drastically. He begins to enjoy causing pain and chaos. He might use his skills not to protect but to dominate and instill fear in others, completely opposite to his original character.
Obviously, Killer Bee is one of the main characters. He is known for his unique fighting style and his role as the Eight - Tails' jinchuriki in the Naruto series. His brother, although not as well - known perhaps in the original series in this specific relationship context, would also be a main character in these fanfictions. Depending on the plot, Naruto might also make an appearance as he has interactions with Killer Bee in the original Naruto story.