Oh hey Anon, I’m sorry this one got lost D:
I imagine that, after Erik’s captured and put on trial and declared guilty and thrown in prison, after he’s had absolutely no one listen to him (including Charles) and no one who would believe him or help him (including Raven), he’s absolutely furious. But it’s not the explosive, destructive rage we see in the beginning of XMFC; it’s the quietly focused rage of the end. We see him sleeping, meditating, and I imagine he’s meditating on… pretty much everything. His mother, his life, Shaw, Charles, Raven, the Brotherhood, how his allies were captured by Trask and experimented on and murdered. He takes the anger from all of that and locks it deep down inside, to save for the day when he can let it out.
Even though we don’t see it, I like to think that Erik kept a diary, written in crayon (no lead pencils allowed, no pens with metal nibs or springs), where he records some of those thoughts. None of the guards even think about confiscating it to see if he’s planning anything nefarious, because the rare times he meets their eyes, when they come with food or clean linens or a clean uniform, they’re surprised to walk away alive. Erik brings it with him when Quicksilver breaks him out, and maybe leaves it on the plane for Charles to find—half-vindication/justification for what he wants to do, half apology.