I have never heard of previous owner's killing a family of four because they've lost their home to a foreclosure. I've heard about owners stealing and trashing but murder, never.
Law enforcement followed the evidence. The evidence lead to Merritt to the exclusion of all others.
I agree it would probably be a first. But the evidence did not form a solidified picture until after the bodies were discovered. What did the investigation entail in the years preceding the conclusion? Do they take a close look at anyone with possible motive?