About the scene where Erik kills Shaw: I don't know about Charles but it always seemed to me that Erik didn't want to betray Charles, he just wanted to enjoy the moment (which would be hard with Charles talking to him all the time) and show Shaw that he is not weak and he doesn't care about Charles (because Erik thinks love is a weakness). He is a big drama queen and he likes to put on a show. He didn't realize he is hurting Charles. I was always wondering if he would act differently if he knew.
Oh! This is also a very good point. You can tell Erik is really relishing the moment. I know he puts on the helmet partially because he’s not sure Charles won’t try to stop him, but there is definitely a piece of him that want this for himself. This is his moment he’s been working toward for years after all. Also a really good point that Erik never wants to show weakness in front of Shaw, and we know if Erik has any weakness at all – it’s Charles.
I do wonder if he knew how much he was hurting Charles. He must know Charles is in Shaw’s mind, but it’s true that there is no way to know how exactly Charles would experience it, but it can’t be pretty. I don’t think he wanted to cause Charles physical pain, however, and I do think he’d be pretty terrified to learn exactly what Charles went through.