It could be that his new 'godlike' power has isolated him. People might be either in awe or afraid of him, and he may feel that he can't be close to others as before. So he acts coldly as a defense mechanism.
Well, at first, people around him would be very confused and try to reach out. But Naruto would just push them away. Then, his former friends might start to get frustrated with his coldness. Some might even turn against him thinking he has changed for the worse.
One possible reason could be a great betrayal. Maybe he discovers that someone he deeply trusted, like a close friend or a respected mentor, has been using him all along. This would shatter his trust and make him withdraw from people, thus becoming cold.
Perhaps he is trying to hide his true self. He may have a side of him that he doesn't want others to discover, like some special abilities or a different origin. So, being cold to everyone helps him keep his secrets and avoid getting too close to people who might figure him out.
One possible plot could be that Naruto experiences a great betrayal, perhaps by someone he deeply trusted in the village. This could make him lose faith in the bonds he once cherished and become cold towards others. He might start to isolate himself and focus only on his own power, believing that relying on others will only lead to more pain.
One possibility could be a great tragedy that befalls him. For example, if all his friends were brutally killed in front of him by an enemy he couldn't defeat at that time. This might shatter his belief in the goodness of the world and drive him to become cold - blooded in order to seek revenge. He would start to see killing as the only way to get justice.
One possible plot could be that after losing many friends in a major battle, Naruto becomes disillusioned. He might start to see the world as a cruel place where strength is the only thing that matters. So he shuts off his emotions to avoid further pain, and becomes cold - hearted towards his former allies and enemies alike.
It could be due to a personal loss that has hit him really hard. Maybe he lost someone very close to him, like Jiraiya all over again, but this time it has broken him in a way that he shuts everyone out. He might also be under the influence of a new power that he doesn't fully understand yet, and this power is affecting his emotions, making him cold as a side - effect until he learns to control it.
If Naruto becomes cold - hearted, his relationships with the villagers would change drastically. The villagers, who once saw him as a hero, would now be faced with a cold and unapproachable Naruto. Children who used to look up to him would be scared. His friends from the Akatsuki days, like Nagato, might even see this new Naruto as a threat. Overall, his relationships would go from being warm and friendly to cold and distant, filled with suspicion and unease.
One possible plot could be that Naruto has faced so many betrayals. Maybe his friends turn on him due to some false accusations. This shatters his warm - hearted nature, and he becomes cold as a defense mechanism. He starts to isolate himself and no longer trusts easily.