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.
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.
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.
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.
The plot would become very dark. Harry would no longer be the hero, so the fight against Voldemort would likely take a different turn. Maybe Voldemort would see Harry as an ally instead of an enemy. The other characters would have to either try to change Harry back or find a new way to defeat Voldemort without Harry's help.
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.
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.
No. Most Naruto fans appreciate the original values of the series, like friendship, perseverance, and kindness. A fanfiction portraying Naruto as heartless goes against the core of the character, so it's not likely to be a common view.