There are two aspects of this story that I would refer to in response to this question. Firstly, I would want to refer to the Wasp Factory itself, which is of course the small space of rambling construction that is based on an old bank clock face which becomes a kind of temple for Frank's weird and wonderful rituals and sacrificial rites. The language that is used to describe Frank's ordering and how he "reads" the omens depending on which death the wasp chooses on the clockface is written in a very matter-of-fact tone. There is no sensationalist language, and Banks does a great job of showing that Frank, in spite of the strange and bizarre activity he engages in, possesses "normal" abilities such as the ability to reflect on his own actions and self-irony.
Secondly, I would refer to the nature of the narrative, which leaps between Frank's own trips around the island, the phone calls he receives from Eric and finally Frank's increasing revelations about his own existence and his past. The violent past of Frank in particular and the way that this is juxtaposed with other "normal" events and happenings again brings the theme of sanity and insanity into sharp focus, as we are forced to question our own ideas of sanity and insanity and we realise that there perhaps is not such a strict dividing line that separates these two states as we would like to think.