I think I get some of it now (spoilers)
I grew up on this film. It scarred the wits out of me especially the last 30 minutes. But the thing was I never understood what the heck happened. It just didn't make sense?? But now I think I have an idea of what was going on. Be warned, spoilers below (if anyone still cares ;)
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The character of David was possessed or was THE First Beal reincarnated (not Jonathan BTW). The David (the unnamed corpse) in the 8th thought to be unmarked grave was actually the original Beal, the one who started everything and was murdering all the family members. He possessed the modern day David who had become obsessed with the Tibetan Book of the Dead. He actually predates Jonathan Beal or maybe he was a younger brother who made a pact to sacrifice 7 of his family members. For what, who knows. That is never made clear. I think the writers screwed it all up by overlapping too many of the scenes, especially with the character of David. I honestly think he may have killed all 7 of his family members. I don't think the wife shot her husband at all and that David Beal killed him. There weren't 2 corpses either, I think it was all one corpse and when David lost it in the cemetery and he fell in the grave he just took on his true form, a guy who'd been dead for the last 100 yrs or so.
The final portrait commissioned by the late and final Mrs. Beal was for the person in the unknown grave. Somehow, she found out who it was. WE find out the identity of the 8th grave when John Ireland's character trips and finds the plaque that is in the ground with David Beal's name and years of life on it. We now know who actually resides in the "unknown" grave maybe this is the same way the last and final Mrs. Beal found out who he was. Or maybe David visited the last Mrs. Beal and flat out told her who he was, I'm not entirely sure how she would create a portrait based on a plaque.
I heard someone say, "Carlos Estevez looks JUST like Charlie Sheen!" Good grief...