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.
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.
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.
The best chapter pointer here is Chapter 86, "If you don't welcome me, why should I stay?". It opens with After over an hour of fuss, it was finally time to eat. and closes on But then her expression became resolute and arrogant..
If you want the clearest chapter reference, go to Chapter 82. Early on, Jiang Li gave her a fleeting glance.; later, the chapter leans on When did we ever make a private deal?" "Just now!" Wen Jiaojiao defended....
Chapter 63 is the strongest chapter-level answer because it starts from During those seven days, he thought he had completely escaped from that... and follows through on It was already past midnight, but the conference room at Xingmei....
If you want the clearest chapter reference, go to Chapter 95. Early on, Hearing others praise her daughter, Aunt Lin was smiling so wide she...; later, the chapter leans on Lingling really is a good child..
If you want the clearest chapter reference, go to Chapter 86. Early on, After over an hour of fuss, it was finally time to eat.; later, the chapter leans on Qi Tianyu, starving, crazily scooped up his curry rice into his mouth..
Look to Chapter 120 for the clearest answer. It begins with Facing the sudden and somewhat strange request from Jiang Li, Director Sun..., keeps building through But, as if by some mysterious force, he still instructed his assistant to..., and lands on Drink this." Wen Jiaojiao looked at the half.
The best chapter pointer here is Chapter 137, "Lai Fu's Death Has Another Cause". It opens with Jiang Li personally went to Lai Fu's tomb and performed a small ritual. and closes on At that moment, Qi Tianyu had just finished his farming tasks, feeling....
A practical answer is Chapter 100, because it turns the idea into a concrete scene. The chapter opens with The barrage was about to die of laughter., adds pressure through Who would dare write a script like this!], and ends on Her eyes flickered slightly as she grabbed the hand..
If you want the clearest chapter reference, go to Chapter 85. Early on, Facing everyone's skepticism and curiosity, Jiang Li explained fluently:...; later, the chapter leans on The tomatoes are grown by them, and the eggs are also from their....
The best chapter pointer here is Chapter 65, "Jiang Li Missing?!". It opens with Several minutes later, the room remained silent. and closes on What's the deal with these two?.