There are situations in which lying can be justified, especially when it comes to saving the life of another human being. Cultists are often extremely fanatical and dangerous individuals, willing to commit heinous acts in the name of their beliefs. Faced with the imminent threat of losing one's life, lying can be a valuable tool to divert the cultist's attention. In life and death situations, the priority must be to preserve the life and limb of the people involved, even if that means resorting to lying.
In a scenario where the alternative is to allow a cultist to exert violence and brutality on his victim, lying becomes a lesser and justified evil, here resorting to lying is the morally correct choice.
RE: I have to keep him alive.