“Rumors.. that 7 more bodies were found”.
I’m not absolutely sure the definition of a serial killer vs a mass murderer; however, this sounds more like a serial killer luring victims via a help wanted ad. If you wanted to rob someone.. why go all the trouble of posting a help wanted ad, making the arrangements to meet, discuss the job, etc... I have the horrible feeling that when all is said and done... a lot of bodies will be found. IMHO.
And why choose to rob people who obviously have very little or no money? If they were going to lure people out there, why not on the pretense of selling the land? Wonder how the police got the robbery theory? Whatever the case, this sounds like a classic horror movie. Sick.