During Erik's joke of a trial, he was allowed a lawyer. It was pointless--he would have rathered they just throw him in prison than waste all of that time with a meaningless courtroom farce, but no one asked him. His lawyer was perfectly coiffed and respectable, an old friend of Charles', a last mercy from an old lover who didn't believe his innocence and didn't bother to attend his trial.
"Solitary confinement...it puts a great stress on the mind," his lawyer told him after sentencing. "You'll see things. Hear things."
Erik scoffed at the time, but spent the next nine years waiting for the moment that Charles would appear to him, angry and bitter and spouting the self-recriminations that lurked in Erik's darkest thoughts. He feared it, yes, but in the echoing emptiness of his bright white cell, a part of him longed for it, too.
He missed Charles. He loved Charles, still--love had never been the problem. Even if he was angry, even if he was only in Erik's mind, having Charles here with him would have dulled the monotony, the silence, the pain of his solitary existence. If having Charles as a hallucination was the only way he could have him, he was willing to make the sacrifice.
Now he understands his folly. He longed for Charles' presence for years, got it back and couldn't flee quickly enough, and here he is, out in the world, unconfined yet still alone with Charles lurking at every turn.
Not truly, of course, rather it's as if Charles left a piece of himself behind when he took control of Erik. That was always Erik's greatest fear, wasn't it? The fear that led Erik to spend considerable time and resources crafting not one, but two helmets to keep him out--that Charles would take over Erik's mind, that Erik's thoughts and actions wouldn't be wholly his own.
He wonders now why it wasn't this that he was afraid of--the constant presence of Charles lingering in his thoughts. No matter how far he runs, how fast, no matter how ruthlessly he acts or how well he hides, he still feels Charles near. Like an after image burned into his mind, Charles is with him always, steadying his hand and reminding him there's another way, there's a better way, he's better than this.
Erik's a rational man. He sees no apparitions. The only things that moved untouched are the ones he controls. He hears no voices, no matter how badly he may wish to. Still, as he stares at the stained ceiling, lying in his empty bed, he knows he's haunted all the same.