Complex sentence: The film's creepiness is enhanced by the the audience knowing the identity of the killers, nearly all of who are friends of Gillis whose wife Janet (Melody Anderson) has suspicious reasons for her own frequent nocturnal disappearances.
Split into simpler sentences: The film's creepiness is enhanced by the fact that the audience knows the identity of the killers, nearly all of whom are friends of Gillis. Moreover, Gillis's wife Janet (Melody Anderson) has suspicious reasons for her own frequent nocturnal disappearances.