I think that the 'murderer of Hinterkaifeck' was Lorenz Schlittenbauer. Here's why:
- He exhibited suspicious activity right after the discovery of the bodies.
- After the demolishment of the farm, Schlittenbauer reportedly mentioned that the killers failed to bury the bodies.
- Seeing as he was the unofficial husband of Viktoria Gabriel, he'd inherit the farm if the family dies.
- Schlittenbauer had an unwavering hatred towards Andreas Gruber.
Let go over each of these points.
After the discovery of the bodies of the Gruber and Gabriel family members, Schlittenbauer, despite being accompanied by two people, his friends Jakob and Michael, entered the main home alone with a key that had been given to him by Viktoria Gabriel. Now, why would he enter the home if he did not know wether the murderer(s) was/were still inside the home?
Furthermore, after the demolishment of the farm, a local teacher spotted Schlittenbauer at the site. Upon the teacher asking him why he was there, he stated that the perpetrators attempted to bury the bodies, but failed as the ground was too hard from the previous winter.
Additionally, Schlittenbauer's alleged relationship with Viktoria Gabriel would, first off, explain how he had a key to enter the house with in the first place, and, second, why he would kill the entire family. But this ties in with the last point, which is that Andreas Gruber and Lorenz Schlittenbauer had a unique hatred for each other.
My theory is that, due to a fallout with Viktoria, parental concerns, among many more things that tie into the points I mentioned above, Schlittenbauer decided to kill the entire family. That way, he'd have nothing to lose.
