Well, it could be due to a misunderstanding. Maybe his friends were deceived by an enemy who made them believe Naruto was a threat to the village. So in a moment of confusion and misinformation, they took his life.
Maybe the story could shift focus to how the world reacts to Naruto's death. There could be chaos as his enemies start to take advantage of his absence, and his friends have to step up in different ways to try and protect what Naruto once fought for. This could lead to character development for his friends as they struggle without him. Also, there could be a search for the true culprit behind the misinformation that led to Naruto's death, which could uncover a larger conspiracy.
In fanfiction, this could be due to an alternate reality or a dark twist in the plot. Maybe Kushina was mind - controlled by some powerful enemy. It's a very strange and dark concept that goes against the normal Naruto storyline where Kushina is Naruto's mother and loves him dearly.
Another possibility in this fanfiction concept could be that Sakura was under some sort of mind control. An enemy might have manipulated her into attacking Naruto. This would create a very tragic and complex storyline, as it would involve the undoing of all the positive bonds they had developed over the years.
In some fanfictions, it could be due to a dark turn of events. Maybe Sasuke, still consumed by his thirst for power and vengeance, sees Naruto as an obstacle in his new, twisted path. Sakura, perhaps being manipulated by Sasuke, joins in. And Kakashi, unable to stop his students' descent into darkness, fails to protect Naruto.
If Naruto killed Tsunade in a fanfiction, it would cause chaos in the Hidden Leaf Village. Tsunade is the Hokage, and her death would leave a power vacuum. Naruto would probably be seen as a traitor by many in the village. His friends might be divided between believing in him and thinking he's gone mad.
In this fanfiction, Naruto and Madara could engage in friendly battles to test each other's strength. These battles wouldn't be about hatred or revenge but rather about mutual growth. Naruto could learn new techniques from Madara, and Madara could be impressed by Naruto's ability to quickly adapt and improve. Their friendship could be based on respect for each other's power and determination.
In a possible fanfiction scenario where Naruto is killed by Gaara, it could be due to a breakdown in their understanding. Maybe a powerful and dark force has corrupted Gaara's mind, making him see Naruto as an enemy when Naruto tries to help. Gaara's sand powers are extremely strong and if he goes on a rampage without realizing who Naruto really is, he could end up killing him.
I'm not entirely sure which specific fanfiction this refers to. It could be a very dark and alternative take on the Naruto and Dragon Ball universes combined. Maybe in this fanfic, there was some sort of misunderstanding or power struggle among the characters that led to Naruto's death at the hands of his friends. However, without reading the actual fanfiction, it's hard to say for sure.
One possibility is that Naruto might have made a great sacrifice that his friends didn't fully understand at first. Maybe he had to go on a dangerous mission alone and was presumed dead. His friends, in their grief and shock, gradually started to forget about him as they moved on with their lives. For example, they could have been so focused on protecting the village from new threats that they didn't have time to dwell on Naruto's absence.
Sometimes in fanfictions, character development takes a dark turn. The friends might have their own personal goals that conflict with Naruto's. They could see Naruto as an obstacle to achieving those goals. Or perhaps there was a long - hidden secret that came to light, which made them change their attitude towards Naruto, leading to the so - called betrayal.