I want to preface this with that fact I haven't, nor do I really plan to, watch all of BNHA or read the entire manga, this is just based on the things I HAVE read or watched, feel free to correct me if I'm wrong on anything.
Okay, the traitor. I did read that Horikoshi apparently actually forgot about this plotline, so there's a chance it will actually never be brought up again, but the latest chapter really got me thinking. Edit: I have been informed this isn't actually true!!! My hope that this theory will come into play is bigger than ever now!
So, in the last few chapters, everyone had a moment with Midoriya, even the smaller side characters like Kouda and Satou. They all got to tell him how they felt about him, how he's helped them at some point in time, ways he's inspired them. Kirishima, Hagakure and Aoyama being the only ones who hadn't, until 322.
Kirishima finally steps in and says what he needs to to Midoriya, bringing up the slime incident and protecting him from falling to the ground and getting hurt.
That makes every single student in Class A, except Hagakure and Aoyama. They are the only two that don't say anything to try to convince Midoriya to come back with them, they didn't get their moment with him.
Now, I don't know a lot about Hagakure, as she hasn't really shown up in the canon content I have seen (I mostly focus on the BakuSquad), but I know it's been a theory for quite some time that she's the traitor, due to certain things that have happened. I actually read the theory before I got into Kirishima (specifically him, he's the only reason I'm here and invested now), but I can't remember the whole theory.
And then we have Aoyama, he's just low-key weird in general, the cheese incident standing out specifically to me. Due to this incident, Aoyama actually became friends with Midoriya, and thinking back to that, it's kinda weird he didn't say anything to convince Midoriya to stay. In the cheese incident he points out that they're similar in the fact their Quirks aren't compatible with their bodies, so I do find it weird this wasn't brought back up in the past few chapters.
I did also read another theory that Aoyama is actually quirkless, and the belt was potentially a gift from the LOV. This does add up with what Aoyama says to Midoriya about wearing the belt since he was young, no one has seen him without it (I don't know how much strength is actually behind this theory though, I just thought it fit with that comment).
In that same couple of chapters, Midoriya makes two comments about Aoyama.
1. None of us have any idea what Aoyama is ever thinking.
And,
2. He doesn't go out of his way to talk to anyone.
Not knowing anything about Aoyama, I thought this was strange, as he definitely gives off a talkative, look at me, kinda personality, so the fact he actually doesn't go out of his way to usually make friends, but fully went out of his way to befriend Midoriya, kinda fishy. Especially as this happens not long after the whole saving Bakugou bit, which is where the villains revealed they knew who All Might had given his power to (right? I vaguely remember the conversation between All for One and All Might).
Then, there is the Class A Vs Mirio battle, in which he just straight up shoots his laser at Mirio's head, this is mostly based on the comments on the YouTube video I watched, so I don't know how much backing it has. Mina also shoots her acid at him, but I read she can control the acidity of her acid, but no one said Aoyama can control the power of his laser, and if that's true, he just straight-up tried to kill Mirio in front of basically his entire class.
And finally, USJ. When the fight is over, and the class are all back in one spot, Aoyama says to Tsuyu, "As for my location, where do you think I was?" To which Tsu asks where and he replies, "it's a secret." Now, sure, this can be seen as him just being weird, but when you mix it with a few other factors of the USJ surrounding him and Hagakure, it seems a little fishy.
For one, the map at the start of the chapter that shows where everyone is, Aoyama and Hagakure are together with a 'where?' under them. They're not given a location.
Later on, Hagakure congratulates Ojiro for holding his own again a villain all by himself, but when asked where SHE was, she says she was with Todoroki. There are a couple of issues with this statement, obviously.
1. How would she have known Ojiro was on his own if she was with Todoroki. The areas that Todoroki and Ojiro were in were on almost opposite sides of the USJ arena, she should have had no way to know Ojiro was on his own.
2. Why would she have not let Todoroki know she was there? Why would she take her shoes and gloves off, and just stand in a place Todoroki could have done her harm by accident. If she WAS with Todoroki, she literally just stood there and did nothing, while putting herself in complete danger of not only the villain Todoroki was fighting, but Todoroki himself.
I find it odd the person she claims to have been with, and the person she knew was alone, were the only people she speaks with. The only two people that were on their own, everyone else was with at least one other person. They were the easiest options to lie about.
We also have to add the why the characters are split up in that scene anyway, Hagakura being with Todoroki wouldn't have even made any sense when you look at how everyone was split.
Kirishima and Bakugou are together, because they were together in front of everyone after duo attacking Kurogiri. Iida, Uraraka, Shouji, Satou, Sero and Mina were standing close together. Momo, Jirou and Kaminari were standing together. Todoroki moved further from the group as it went out, so he was alone. Midoriya, Froppy and Mineta were almost standing together. And then there's Ojiro and Aoyama. Aoyama was directly next to him, so isn't it odd, following the pattern from the rest of the students, that Aoyama wasn't with Ojiro? Instead his location was unknown. And then Hagakure, she was directly in front of Satou, so realistically she should have been with that group as well, but she wasn't. She was apparently with Todoroki, which just doesn't fit the pattern, especially as Todoroki throws himself further from the group as they're teleported, which is why he was alone in the first place.
Everyone who was sent off together, are standing together. Hagakure can be seen standing with Satou and Shouji, she definitely should have been with them.
So, the fact both Aoyama and Hagakure's actual whereabouts during this attack are unknown, seems really odd all things considered.
I then come to the Training Camp situation, the only other villains attack on the school I know anything about (was there another one?). Once again, Aoyama and Hagakure are together in this arc. Sure, Hagakure is passed out, and Aoyama is hiding behind a rock but just the fact they're inexplicably together AGAIN in a villain attack, weird.
And finally, back to the latest arc, which isn't technically a villain attack arc, but again, they're together. When Midoriya let's the white smoke off, Kirishima, Aoyama and Hagakure are split from everyone else. Kirishima makes up for this, he plays his part, helps save Midoriya, but Aoyama and Hagakure don't. They both agree when Kirishima says he has things he wants to tell Midoriya, but they don't. They don't say anything. Even Mina gets a second attempt at convincing him, but Aoyama and Hagakure say nothing. They could have shouted something before Bakugou's apology, but they don't.
There may be things within the stuff I haven't seen or read that disproves this theory, but I think, from these facts alone, both Hagakure and Aoyama being the traitors TOGETHER makes a lot of sense in my opinion.