In the comics, Negan is responsible for the deaths of multiple characters. For instance, he brutally kills Glenn and Abraham, which had a huge impact on the story.
Thanos kills a bunch of characters in the comics. He's notorious for eliminating powerful heroes and causing chaos. It's hard to list them all precisely, but it's a significant number.
It might also be a very extreme statement. Some people are very protective of the original Naruto work and view fanfiction as a sort of 'corruption'. However, fanfiction can also be a form of creative expression for fans who love Naruto and want to explore different scenarios or relationships within the Naruto universe.
Well, often in fanfiction, a post - kill Naruto could turn more ruthless. Having crossed the line of taking a life, he may see violence as a more viable option in future situations. This change might also make him more distrustful of others, fearing that they might also be capable of such extreme actions. He may start to train harder, not just for the sake of protecting but also to be able to better defend himself against potential threats.
It depends on the specific fanfic. Some authors might come up with this plot twist to explore different character motivations and conflicts. In others, it might not be the case at all.