She and the community that knew the family needs to come to terms with the fact that her father snapped. Imagining conspiracies and complicated alternate theories isnt helping her heal. Her father isnt the first seemingly normal person to snap and he wont be the last.
And frankly I get tired of the line, "Oh but they were so happy and normal when I saw them! How could such a normal happy person do something so horrible?!". Its such a stupid narrow minded thought process. People are complex and whats on the surface is rarely the whole picture. People are like icebergs. There is the 10% above the surface that most people see, but below is the other 90% that few people actualy see. That 90% is the actual person. The real mass of who the person really is.
How many times have we seen seemingly normal people snap? Every single year it happens. And we have seen seemingly normal people turn out to be absolute monsters. Its not some inpossible thing. Humans do this. It happens. Its horrible, but it does happen.