I think the Zenin Massacre was the first sign of JJK's writing falling off.
Overall I enjoyed the zenin massacre arc but I think it could've been so much more, it felt like this was the pivotal moment of Maki's development but its just over so quickly I dont feel that. I liked maki getting her power up and destroying everyone but I dont really feel much attachment or weight with any of the villains she's killing. Naoya is introduced a few chapters prior and hes not really developed or explored all that well imo. I get hes a representation of the zenin clans values but hes so over the top and on the nose about it, it kinda becomes cartoonish. Im not saying hes a terrible villain I just feel like he would serve better as a henchmen. I think the main villain should've been an evil bastard as well just with more aspects to their chatacter and a stronger connection to maki like her father who is the first person she kills. The other zenin clan members are literally nothing, we've never been introduced to them before so I dont really care much when they get destroyed, mai's death was fine it did kinda feel like she got shafted in the plot leading up to it however. Finally the zenin clan getting destroyed doesnt really leave any impact on jujustu society when it feels like it really should've. (also I know naoya comes back but it still doesn't solve my issues)
This is just my opinion though, id like to see if anyone else agrees or if you wanna prove me wrong.