While this would seem like a very bizarre casting on the surface, the fact is that Mister Sinister is an integral part of Apocalypse's story in the comic books and, if he is going to be in X-Men: Apocalypse, there's nothing to rule Ben Hardy out of playing him. Mister Sinister is Nathaniel Essex - a man who made a pact with Apocalypse to mutate him into a powerful, immortal being with various powers (including the likes of telepathy, telekinesis and superhuman strength and durability), and worked with the powerful blue mutant before turning on him (in fact, it was Mister Sinister who manipulated the creation of Cable specifically in order to use him to destroy Apocalypse). Hardy is far more suited to other roles, but until Sinister's inclusion is confirmed and an actor cast, nobody can be sure that Fox and Marvel haven't made a bold move in casting the Eastenders star in the role. And who can say it wouldn't turn out to be a stroke of genius if they have?