Perhaps Sasuke had made a deal with some dark forces that Naruto couldn't allow to come to fruition. Naruto, being the protector of the village and his friends, saw killing Sasuke as the only option to break the deal and save the day. He may have weighed the life of Sasuke against the safety of the village and the world at large and made the heart - wrenching choice.
It could also be due to a misunderstanding. In the heat of the moment during the vote, there might have been miscommunications or false assumptions. Naruto might have thought Sasuke was going to betray them all and in his haste to prevent that, made the tragic decision to kill him.
One possible reason could be that the council has been secretly plotting against Naruto all along, perhaps trying to undermine his power or even get him exiled. Naruto might have discovered their malicious plans and decided to take pre - emptive action to protect himself and his friends.
Power imbalance could also be a reason. If Naruto's power suddenly goes out of control or he misjudges the Teen Titans' power levels and thinks they are a danger to his world or friends, he might take extreme actions. For example, if there's some kind of power - up in Naruto that makes him overly confident or paranoid, he could lash out at the Teen Titans thinking he's protecting something important.
One reason could be that Sasuke realizes he needs Naruto's unique abilities. Naruto has the Nine - Tails' power which could be useful in a certain task. So he takes Naruto with him to achieve a common goal.
One possible reason could be a sudden and extreme corruption of Naruto's character. Maybe he is influenced by some dark and powerful force that makes him turn against his friends. Another reason might be a misinterpretation of a prophecy or some kind of mind - control situation.
One possible reason could be that Sasuke might have gone too far in his pursuit of power even after the war. He could have been a threat to the newfound peace in the ninja world, and Naruto, as the protector of peace, might have felt he had no other choice.
Well, it completely shatters it. Their relationship which was built on rivalry, friendship, and a sense of mutual understanding is now gone. It's a tragic end to what could have been a great story of redemption and friendship.
It could also be that the situation was extremely dire. For example, the troll was about to destroy something very important, like a magical artifact that could cause great harm if it fell into the wrong hands, and Hermione saw killing the troll as the only way to prevent that.
Yes, such fanfictions exist. In these fictional scenarios, Naruto might kill himself because of Sasuke for several reasons. Naruto's whole life has been centered around Sasuke to some extent. If he feels that he has failed in his mission to bring Sasuke back to the right path and that the situation has become so dire that the only way to atone for his 'failure' is through his death, he might do it. Also, if Sasuke were to cause harm to someone Naruto holds dear, like the village or his other friends, and Naruto feels powerless to stop him in any other way, he could choose to end his own life as a last resort.
One possible reason could be a miscommunication that escalates out of control. Maybe Sasuke is on a path that Naruto believes is too dangerous for the world, and Naruto, in an attempt to stop him, accidentally kills him. For example, Sasuke might be about to use a powerful and uncontrollable jutsu that Naruto fears will destroy the Uchia compound and much more, so he acts in haste.
Maybe in a very dark and twisted fanfic, Naruto could be possessed by some evil force. This would override his normal sense of self and his love for his friends, causing him to harm them.