Perhaps a dark power corrupts him. In the fanfiction, there could be some sort of dark chakra or evil entity that latches onto Naruto. This entity could slowly eat away at his emotions, starting with his heart, making him gradually lose his ability to feel empathy and turn heartless.
Maybe Naruto has been through some extremely cruel experiences that make him lose his compassion. For example, he might have witnessed the repeated betrayal of his friends, which gradually hardens his heart.
It could be Sasuke. If Sasuke has turned completely to the dark side and Naruto fails to save him, Naruto might be so frustrated and angry that it starts to make him cold - hearted towards everyone. Sasuke's actions could be the trigger for Naruto's transformation.
One challenge could be that the heartless have a very different nature compared to the beings Naruto is used to dealing with. Their lack of emotions might make it difficult for him to find a way to connect and control them.
Well, in some fanfictions, Naruto might control the heartless through his strong will and the power of his chakra. Maybe he uses some new jutsu he developed specifically for this purpose.
Perhaps the discovery of some dark secrets about the ninja world could make him ruthless. For example, if he finds out that the village leaders have been using him or his friends for some immoral purposes all along. This new knowledge could shatter his world view and make him question all his previous beliefs, leading him to become a more ruthless individual who is out to change or even destroy the existing order.
He becomes cold towards others. He no longer shows the warm smiles and friendly greetings that he used to. Instead, he might ignore people or give them cold stares.
One possible reason could be a traumatic event. Maybe he witnessed something so horrible that it made him shut down his emotions to protect himself. For example, if he saw all his friends killed in front of him, it could be too much for him to bear and he just numbs himself.
Well, it could imply that the fanfiction about Naruto is lacking in emotional depth or empathy. Maybe it portrays the characters in a cold or unfeeling way, not in line with the original spirit of the Naruto series.
No. Naruto is known for his kind heart and strong sense of justice. In the original series, he always tries to understand his opponents and save them rather than being a heartless killer.
It could be the influence of a powerful enemy or a dark power. Suppose there is an enemy who has the ability to manipulate emotions or a dark power that corrupts Naruto's mind. This could lead him to change his personality and become more like Gaara. For example, if the enemy makes Naruto feel alone and unloved, he may start to develop the same cold and distant behavior that Gaara once had.