Who we can exclude from being the traitor:
Bakugo, Shoto, Ochako, Iida, Yaoyorozu, Kirishima.
Kaminari, Jirou, Ashido (fought the League and helped poison Giganto, and we saw Kaminari's thoughts, he loves Jirou + Ashido only remembered Giganto when she saw him in the forest).
Tokoyami, Mezo, Tsuyu (had fights with the League and Shigaraki would have decayed Tsuyu, no way he'd do that to a friend).
Literally the only possibilities would be guys like Sero, Koji and Hagakure. And Aoyama was semi-debunked already. I don't see any room for a traitor in Class A.