@dully There's a lot of signs actually. Here are the ones I've identified.
1) The lines "nobody will ever love you except for me". I forgot the exact words that Cedric said, but it was nearly something identical. That's an IMMEDIATE red flag and signals gaslighting in the relationship towards the victim. One of the things that abusive partners do in domestic violence is instill the belief in the victim that they are useless and unwanted outside their abuser (which is why victims stay in the relationship and don't find help), and "I'm the only one who will ever love you/you're worthless to everybody else" is almost always a constant if you look at domestic abuse cases. Literally if your romantic partner says that to you, hit the floor running.
2) The minute Aileen asserted her own independence outside Cedric is the moment he flipped. He immediately turned physically violent (even if he didn't hit her, grabbing someone like that counts as physical aggression). To me, this looked like it had to do with ownership. He obviously thought he completely owned Aileen and hated that she had a shred of her own thought. The case with domestic abusers is that they cut off their partners from everybody else so that their partner becomes reliant on them. And you can guess, they flip if they discover their partner is interacting with other people (who could potentially help the victim out of the situation).
3) and finally his last line. The whole thing about "betrayal"... not sure how to explain this one, but that one just screams the makings of a domestic abuser to me.
Anyways, if you wanna read up on domestic violence, "Why Does He Do That? Inside the Minds of Angry and Controlling Men" is a hella good read. It's obvs focused on male abusers, but that happens to be the case with a lot of these situations.
But then again, this is manga and a lot of people are calling it just yandere. Maybe I'm too realistic lol 🤷♀️