Ending explained! (SPOILERS)
It seems nobody posting here completely figured out the ending to the movie, so lemme take a shot....
First of all, realtor Jake was actually Todd Monroe, the third occupant of the house in the prologue. His glasses were broken the day before the film begins, when he helped murder a trio of innocents (though admittedly, you'd think he'd have replaced them 20 years later). After the shoot-out the Monroes changed their identities and Jake seemingly went legit by becoming a realtor (much easier to get away with faking credentials in those days). This afforded him the opportunity to buy back the house.... and earn an illegit living. He'd sell the house to a new couple every few years and split the dough with his poverty-stricken sister-in-law Anna/Olie and brother Dwayne, who would in turn kill each of the new tenants. Regular profits off of one property.
In flashback, all of the other victims were completely blindsided by Dwayne, so it seems a bit incongruous that Olie went out of her way to cozy up to the new residents, told the story of the last people's deaths, and provided Ben and Ruth with a gun -- and Dwayne spent an inordinate amount of time toying with Ruth when he could have easily killed her and been done with it. Matter of fact, when he finally corners her, Dwayne assures Ruth that he won't hurt her, and for the first time he prepares to rape one of his victims. Easily explained, as the plan had changed...
It's clear pretty much from the get-go that Jake had romantic designs on Ruth. So off-screen, Jake went to his former roomies and laid out a plan wherein they'd drive Ruth to the brink of madness, at which point she'd kill Ben -- and she'd be so distraught over the ordeal that she'd find solace in Jake's arms. Explains why Olie didn't answer the door/phone. His master plan was that Olie and Dwayne would take the fall for everything, which is why Jake arrived at their house brandishing a gun and he had no qualms whatsoever about killing his brother.... thus leaving him with a beautiful widow for a bride, no one to expose his real identity and two less mouths to feed. Or perhaps he was just pissed at Dwayne for trying to rape his future wife, whom Dwayne probably wanted simply because she was earmarked for his brother. Maybe it was a combo of all of the above.
No longer obligated to provide for Dwayne and Olie, Jake gave up the money-making scheme that Olie had conceived. He didn't show the house to the Bumfords, they discovered it on their own, so he somewhat reluctantly sold it to them. But when his beloved and now-crazy Ruth protested to anyone else living there, Jake dutifully pushed Mrs. Bumford into the well and later hanged her hubby. Or maybe Ruth murdered the Bumfords on her own, since Jake didn't have much of a penchant for murder (he only killed the first time to conceal his identity) and she'd clearly gone off the deep end by that point. Wouldn't have been difficult to push the old lady into the well and the old man was a little dude.
At least that's the way I interpreted it. I thought this was a great film and find it annoying that everyone's saying the ending sucked and/or was illogical because every detail wasn't explicitly spelled out. I think a big part of the problem was that they used flashbacks at the end (doubtlessly enforced by the studio) to explain part of the story, so audiences expected a complete explanation. Sometimes you've gotta use your brain a little bit... which is part of what I liked about this low-key little flick.