Well, perhaps in this fanfiction, Naruto's family was brainwashed by an enemy. They no longer recognized Naruto as their own and instead saw him as a threat to their new 'false' ideology. So they decided to kill him. This would completely shatter Naruto's image of family as something warm and loving, and would be a really cruel take on the Naruto universe.
Well, one possibility could be chaos in the ninja world. Naruto is a symbol of hope and peace. His death might lead to power struggles among different villages as they try to fill the void. Villains could become emboldened and start new attacks.
Well, it would be a really dark plot. Maybe Sasuke, consumed by his thirst for power or revenge, finally overcomes Naruto in a brutal battle. This could lead to a world in chaos as Naruto was the hope for many. The Hidden Leaf Village would likely be in a state of despair, with all the friends of Naruto vowing revenge on Sasuke.
It could be that the family discovers a false prophecy about Naruto bringing doom. In the fanfiction, they might be so desperate to prevent this 'doom' that they decide to kill Naruto, not realizing it was a false prophecy all along. This would add a tragic element as their fear led them to commit an unthinkable act.
One possible plot could be that during a very intense and dangerous training session, Jiraiya attempts a new and powerful jutsu. Naruto, being eager to learn and prove himself, jumps in too soon to assist or observe closely. Jiraiya loses control of the jutsu slightly and it accidentally hits Naruto, leading to his death. This would then cause a huge upheaval in the ninja world. All of Naruto's friends and allies would be in shock and disbelief. Sasuke might be filled with rage and vow to seek revenge, not fully understanding it was an accident at first. And Jiraiya himself would be consumed by guilt, perhaps even going into a self - imposed exile as he couldn't bear to face the consequences of what he had done.
One possible plot could be a dark turn in the story. Maybe Kakashi is under some kind of mind - control. He could be forced to fight Naruto by a powerful enemy who has taken over his mind. Naruto, being unprepared for such an attack from his mentor, gets killed. This would then lead to a series of events like Sasuke going on a rampage to find out the truth and avenge Naruto, and the whole ninja world being in shock as one of their most promising heroes is gone.
The most immediate consequence would be the reaction from Naruto's friends. Sasuke, for example, would likely go on a quest for revenge. He has a deep bond with Naruto and would not let his death go unpunished. Sakura would also be devastated and might lose faith in the idea of peace between the villages.
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 scenario, it could be that an enemy uses some sort of mind - control jutsu on Naruto's family. The family, under the control of this jutsu, doesn't recognize Naruto as their own and kills him. This would add an element of tragedy as Naruto would be killed by those he loves and trusts the most without them being fully in control of their actions. It would also open up the story for a revenge plot by Naruto's friends or other allies against the enemy who controlled his family.
His father, Minato, could be a likely candidate in a really dark fanfiction. Maybe he was resurrected in an imperfect way and his mind was warped, causing him to turn on Naruto.
Well, it could lead to chaos in the Hidden Leaf. His friends like Sasuke might go on a vengeful rampage. They would be filled with rage and guilt, thinking they could have prevented it. The village would also be in turmoil, as Naruto was their hero and a symbol of hope.