This perfectly illustrates why I always say that Hori is a really bad writer. How many readers actually cared about Aoyama? He was never even shown to be especially liked in his own class. Unlike say a character like Tsuyu which has been mentioned by other people to be "the heart" of her class, Aoyama was always just a background character... and a joke character at that. heck, there being a traitor didn't even help the villains any at all... instead it was used to help the heroes... which is just dumb. We all know the heroes are going to win and at must have a side-character at most die... so there is zero anticipation here. The only characters that we as reader can worry about are the villains, and even then we all know that they are going to lose badly and that Hori is going to try and use them to make his hero characters look good.
I was fine with Aoyama being revealed as the traitor on the principle that he was easily the most suspicious character, but I feel like more should have been done to set him up.
I think it would have been really pertinent to emphasize his connection with Deku as well as his heroism leading up to the war. Make it clear that "yeah, Deku and this guy are good pals". Maybe flesh out Aoyama's interactions with the people from his work studies like Hagakure and Mina, that way, Hagakure being so emotional about Aoyama's reveal doesn't feel like literally the only character moment that Hagakure has ever truly had.
There was the set up for something good with Aoyama, and it looked like Hori was going to do something with his and Deku's friendship during the culture festival, but it was pretty much forgotten about after that.
Also, I'm in the realm that thinks that Hori may not even be gutsy enough to kill off the side characters.
The worst casualty of the war was Midnight, and literally no one cared about that. Even the character don't give a crap about the loss of Midnight.
At this point, the characters I'm most expecting to perish are the villains, and its not like the story has set up the villains as terribly sympathetic at the moment. Yes, they have sympathetic backgrounds, but this arc has gone out of its way to have them double down on their evil motivations, which is fine, but it makes it difficult to punctuate the "tragedy" of them losing. The villains don't have anyone as sympathetic as Twice on their side anymore and I think they suffer because of it.