I think the problem is more that Erik has been broken so many times that he cannot help but overreact. He's very aware that what he's doing isn't right, but he's lost so much that he has drawn a line in the sand and said 'what I have now is mine and is staying that way.' He can't break that cycle of desperate need to keep somebody alive.

It's interesting actually how you interpreted the diamond and pearl, because while I was thinking of it as their relationship, you've seen it as Charles, which is a really fascinating alternative, because both work. Hmm. (In the most egotistical way possible I have to say that is one of my favourite metaphors in the fic. I really like it.)