rooster, you just described my childhood. I grew up very wealthy, with no friends, and stayed in my room all the time. The problem is you're assuming most children/young adults are similar, when they're not. I am relavity the same as I was back then as well, many people I know contribute this to asbergers syndrome but I have never been properly diagnosed with anything.
I still haven't legit thought about murdering anyone. Even when I'm upset. I've considered hurting people, and have laughed at the prospect of some people around me getting hurt... But I've never gone through with it, and it never comes to the point where I am honestly contemplating killing someone.
I think we are projecting what we think children/young adults should act and think like, but really we have no idea. I think attributing mass murder without justification to mental illness is the best explanation we have at this point. However, there might be a genetic aspect to it as well, but I think that ventures into the realm of pseudo-science.
Maybe some people are genetically predisposed to murder. Perhaps many people, such as yourself, have those thoughts because of that. Perhaps societal pressures do eventually cause these types of people to crack and commit murder, but I'm just guessing at this point. I think most everyone is guessing about this, because most people inherently think its evil and don't understand it.