What would be the motive? Why would anyone want to kill or hurt the child?
Again, no history in RZ's background to indicate violent tendencies or suicidal tendencies.
However, there is clear violent tendencies in JS and DS's backgrounds. The death of a child is a huge motive for murder.
A child falliing from a staircase is not uncommon but RZ's death is uncommon.
The intent to murder does not have to be present for a death to nonetheless be the result of negligence. The child's death was preventable. RZ's suicide is not only common but it was also ruled a suicide.
I've yet to hear of a case where parents of a child who is still alive didn't wait for LE to investigate and decided to murder in retaliation. I think it is a rather far-fetched theory in this particular case because it was crudely staged to appear to be a homicide.
JMO