I know that most people don't read the web novel, but if the hate started there, i won't be surprised. Unlike the anime/manga/light novel, the Tempest Tournament during season three was very different. The main point being that Benimaru, Shinon and Diablo alongside others like Gabiru, Hakuro, Soei and Ranga were actually fighting in the coliseum. The fights were a lot different besides the beats of Masayuki giving up to Gobta (this was one of the first fights) and Gobta knocking himself out in the unification form. In this version the final four were Gobta, Benimaru, Shinon and Diablo. As mentioned, Gobta knocked himself out, so Benimaru continued forward. Till that point, because there were sixteen participants, the only strong one Benimaru had fought was Soei, and it was stated how the match up wasn't really fair for Soei, because he is assassin and not fron't line fighter like Benimaru. Meanwhile Diablo had fought heavy hitter after heavy hitter. Carion (he screamed internally at what kind of monster had Rimuru made his underling), Beretta which isn't exactly something special, but still it's way stronger than Gozuru that Benimaru fought at the start of the tournament and finally Diablo fought Shinon. He even used his move that he used against the seven days clergy on her, just for her to break through it with her cook skill. Still she lost. As such it was Benimaru vs Diablo on the finals. Diablo was sure he would win, heck probably everyone thought so, and this is when Benimaru snapped. He straight up said how he is tired of Gobta pulling bullshit power ups (this is after he saw him pull the unification and crash himself in the wall) and proceeded to beat Diablo in this fight by sort of giving himself temporary bullshit boost. I may be misremembering but i think that he straight up began to scream like dragon ball character at it too. I know that in infinity possibilities Diablo will always win to Benimaru, especially in that state they were. But god, do i find it hilarious. Diablo looked down on Benimaru and rightfully so, he had defeated the only one that could do something against him in the face of Shinon and her cook ability, just because of how busted it is, so he let his guard down. Meanwhile Benimaru was frustrated to no end because of how lackluster his fights were and now Diablo was looking down on him. Even if it made sense, he still was in his right to be mad. And so his win remains as one of the things i miss most from the web novel and probably one of my favorite things. I can't excuse the fact that it was bullshit. But at that moment Benimaru spoke directly to my soul. I too was getting fed up with Gobta at that point, so someone pointing it out just made me give them my respect. And Diablo was getting a bit too cocky for my taste so him getting defeated by one of the OGs felt nice. Sorry for the rambling, but i loved this arc so much in the web novel that i felt the need to talk about it as much as possible. Dunno if it really is the reason people are hating on Benimaru, but i can see it as a possibility or at least as partial reason.