Should I warn you?

An old and familiar nightmare resurfaced last night, one I particularly dislike. I didn't scream fortunately, but this one was always disturbing to me for a rather strange reason: I used to believe it was real. It's the kind of belief that's possible only when other impossible things are believed, such as Satan, demons, the spirit realm and such. As a teen and young adult,  I thought this thing, this itinerant entity was real though I never told anyone else. When one of Jehovah's Witnesses believes in the existence of something so specific as this, and it's never been mentioned before by the Governing Body, that person could be suspected of having read unapproved outside material. I couldn't stand the thought of an  inquisition by the congregation elders- nobody can- so I kept this knowledge to myself. I don't at all remember how this wandering entity came about in my inner universe. It wasn't a fallen angel like the demons and Satan. I never knew where it came from, only that it was the spirit equivalent of a malicious animal, hunting for prey as it moved around the earth in its inscrutible orbit. It had a two-syllable name beginning with Na- something. I've forgotten. "Na****sh" or Na****esh" or "Na****azh" or whatever. I don't know, don't care.
But last night I awoke from that nightmare in a hazy, not-fully-conscious state of mind. I pondered over the three-meter long snake-like shadow that was Na****, sensing it's presence out there in the city, not knowing if it's going to make a little detour during its obscure journey by paying me a visit. Either to just taunt me like it has before or to take my life this time. My first thought was concern for my cats: it wouldn't kill them if it killed me, or it might kill them only just to torment me. Leaving one dead the others traumatized, "marked" for life is its modus operandi; it never chooses to take everyone in a household. Then I thought: what about the family I live with?
I was finally awake at this point, greatly relieved by the assurance that Na**** really is just a fictious shadow-being. But I thought this through nonetheless: If I had solid reason to believe this thing was real, should I warn the Tran family? It could only be passing through this region, unaware or unconcerned about my being here. Or it may pay me a visit only to scare me, furthering my trauma then leave. But if it did kill one or more of them, I would certainly have some share of guilt for not warning them to stay away for a night or two. But they would never have believed me if I did warn them, so the result would be the same, so what's the point of warning them?
A mandate imposed on every member of my former religion is to go out and warn as many people as each one can of the impending war of Armageddon. Whether worldly people choose to believe us or not (as if belief was a choice ( °~°)) was mostly irrelevant. If they died without us warning them, we're blood guilty, unqualified to live forever. If we did warn them and they died anyway, then we've done our duty and have a shot at living forever in paradise on earth. Believe me, I was never comfortable preaching this "Join us or die" message. So, In this make-believe situation I'm playing with, should I warn them of the possibility that maybe an evil shadow will slip into our home and kill one or more of us? Or at least inflict life-long trauma on one of the girls without killing anyone? Either way the outcome is the same.
I gotta tell you, the discovery that spirit-beings, a vengeful god and the rest are fiction is like the first bowel movement after a lifetime of constipation. Yes the process is agonizing, but when it's over, you feel so much lighter and totally relieved the nightmare is gone!