Rewatching past episodes, Mib stated that Jacob stole his body (or at least I think that's what he said? It was last night and I really don't recall his exact words so correct me if I'm wrong here)
Soooooo what if Christian was the body Jacob stole. We all know MiB has taken the form of Eko's brother (name escapes me atm), Alex, Richard's Wife, Locke, a horse, etc etc etc. He likes taking the forms of other people/things for his own pleasure (or good or whatever) so what if he was mad @ Jacob for taking Christian's body?
I may be speculating wayyyyyyyy too much here, but oh well.