Well, perhaps in this 'harry kills everyone fanfiction', there is an alternate reality where Harry has a different upbringing. He might grow up in a more hostile environment that warps his personality and values. He could be part of a group that has extreme views and they convince him that the only way to bring about change or gain power is to eliminate everyone who stands in his way.
One possibility could be that Harry is under some dark magic influence. Maybe he's been corrupted by a powerful and evil spell or artifact that makes him lose his sense of right and wrong, leading him to turn on everyone. Another aspect could be that he's been betrayed so deeply by those he trusted that he snaps and decides to take revenge on everyone, believing they are all against him.
In this fanfiction, it could start with a great tragedy that befalls Harry. For example, he might lose all his friends and loved ones due to a series of events that he blames on the wizarding world at large. This overwhelming grief and anger could drive him mad. He may start to see everyone as the enemy, and with his powerful magic skills, he decides to go on a killing spree. It could also be that there is a prophecy that he misinterprets which leads him to believe that killing everyone is his destiny.
Look to Chapter 170 for the clearest answer. It begins with Watching the two who had long remained silent, the PD shifted his position..., keeps building through If you don't set off soon, you'll probably miss lunch." Hearing this, Su..., and lands on [Didn't the other guests also do farm work in.
A practical answer is Chapter 189, because it turns the idea into a concrete scene. The chapter opens with Seeing his painstakingly planted roses trashed like this, even as a fan of..., adds pressure through Meanwhile, a rare new message popped up in their small group chat— "How..., and ends on.
Look to Chapter 165 for the clearest answer. It begins with How come everyone else got a solid six points, and he ended up with just..., keeps building through Director Sun spread his hands, "Our staff calculated the total score with..., and lands on Various explosive Weibos sprang up like.
The best chapter pointer here is Chapter 183, "Sacrifice Oneself, Break Them Up!". It opens with Seeing that he held a blue ball identical to Su Yinwan's, Jiang Yan... and closes on Hadn't this woman been relentlessly pursuing him before?.
Chapter 187 is the strongest chapter-level answer because it starts from During the competition, she passed by him numerous times, unfazed by... and follows through on Perhaps it was a pig that tripped you; they are quite aggressive, after....
A practical answer is Chapter 200, because it turns the idea into a concrete scene. The chapter opens with Shang dismissed the thought of turning around and walking away., adds pressure through She scoffed in front of the door at this man's hypocrisy, then left., and ends on Are you doing this on.
A practical answer is Chapter 181, because it turns the idea into a concrete scene. The chapter opens with Xiang Hao rushed to the police station as soon as he saw the trending..., adds pressure through Next time the production team is selecting members, please be more..., and ends on Let's not be.
Chapter 164 is the strongest chapter-level answer because it starts from Damn, the production team is so shady, they're still doing a live... and follows through on The current Jiang Li did indeed make him see her in a new light, to some....
The best chapter pointer here is Chapter 144, "She's Too Bullying!!!". It opens with Ordinary people would have been at a loss in this situation by now. and closes on He then noticed the hand—fair and slender, not at all like an auntie's..