Why would a young man suddenly decide to become a mass murderer?
It's for a novel I decided I would try to write.
The entire thing is this relatively young man probably early to mid twenties starts murdering large families. I'm sort of thinking that perhaps he was part of a large family as a child, but what could happen that would be severe enough to traumatize a young boy so much as to make him become a sociopathic, mass-murder?
Well it isn't likely that he just woke up one day and suddenly decided to kill people. It is more likely, like you said, he came from a large family, might have been the oldest son, was abused by a father who was a drunk and beat him severely. The father was a drunk, so that is where the narcissistic personality disorder comes in for the father and the son. You are likely to be narcissistic before becoming a sociopath. The boy may have also taken beatings from the father for things the younger siblings did. The father put the boy down and always insulted him and told him he was worthless and good for nothing. This boy was not raised in a loving home and was never encouraged or loved. He would have started drinking, probably around 12-years-old. When he joined the military he got hooked on drugs and the drugs is what caused him to become a serial killer.