Why would a grow man think that his adopted child would entertain thoughts of killing him and his wife?

and the child was not a bad child, but a quiet young girl who may have been troubled and angry about the abuse and neglect that was inflicted on her in the past, but was otherwise innocent and non violent? Do you think that constitutes paranoid thinking, or is it normal for a parent to think like this sometimes? is there something wrong with someone who thinks like this? ALSO, for the parents to snoop out notes and papers that the child has written for any mental abnormalities the child has? is that ok?