To give the sheriff the benefit of the doubt, I think he was in a bit of a shock at the crime scene and didn't really know how to react. This is small town, things like this just don't happen there. (As a side note, my mother is 85 and her house is immaculately clean. She does not have hired help.)
Now, I'm going out on a stretch with a scenario. I think I've watched too many episodes of Criminal Minds.
Is there any way to research if there have been any attacks on elderly women in the area? Any rapes, kidnappings, break-ins or attempts at such things? Or a cluster of such things anyplace, not just locally?
Could it be a predator that preys on elderly women? Maybe she was the target and Russell just happened to get in the way. His head was taken as a trophy or as some sick way to control his prey.