This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
The Chronicles of the Imaginarium Geographica
Charles's treatment of Magwich is the result of him being a Loving Bully. It would explain why he always feels the need to mention (i.e. insult— think of it as "Have I Mentioned That I Hate Magwich Today?") him when he's not around and volunteer to keep an eye him, even when no one asks. Only, instead of dipping his pigtails in ink, he threatens to mutilate him with hedge clippers. It also helps that fate keeps forcing them to be together. Or maybe that's just mutual karma.
Caretakers of the Imaginarium Geographica since John, Jack and Charles