There is no way that a human can harbor that kind of evil without telegraphing it in some way.
This. A thousand times this. I am so sick of this "Nobody could have seen this coming" BS in situations like this. Anyone who could shoot an entire classroom full of small children in cold blood is either evil on a scale hard to fathom or completely batchit insane. Maybe a combination of the two. But no matter which of those best describe this guy there is NO WAY a person like that could spend 18 years in the same house with other humans without them noticing some serious warning signs. I have a 15 year old son. The notion that he could secretly be the kind of person that could do something like this without me having any clue is laughable. This is not like some teenager slipping behind your back and drinking beer or smoking cigarettes without you as a parent knowing about it. There is NO WAY a person capable of this kind of insanely evil act is getting up every day and leading a seemingly normal life.
