It is completely reasonable given the brutality of this crime that this was not a parent.
*He then proceeded to bludgeon his children to death.
*It was soon discovered that Mary had been shot, and that all three of the victims (his kids) had their throats slit prior to the explosion.
*She had deliberately rolled the car into the lake to drown them.
*He then proceeded to attack his sons with a hatchet before blowing up his house in a premeditated murder-suicide.
http://listverse.com/2013/04/21/10-shocking-cases-of-parents-murdering-their-families/
Sure it's still completely reasonable that because of the brutality it wasn't a parent.