That is an astounding leap of non-logic. My first point was that the little punk couldn’t have stockpiled his weapons if he hadn’t had privacy. My second was that if he’d been part of a close family he might not have had the urge to stockpile them in the first place.
The child was sick, that I do agree with..I don't know what kind of relationship his family had. Even if he did have siblings, he probably wouldn't have been sharing a room, I just find that an 'astounding' leap as well!!
And if he was sharing a room he would have probably found out another way to carry out his plan.