I really don't think we can attribute this murder to just one set of circumstances, i.e., because he was dumped by Emma, or because he had a horrible home life, or because he listened to and lived the culture of horrorcore. Like most murders, I think, and particularly an especially gruesome, multiple murder such as this, they result from a "perfect storm" of circumstances, so to speak. I don't want to get into a debate about nature vs nurture, but maybe it's safe to say Sam was born with some kind of genetic predisposition to violence and aggression, judging from the words and actions of other members of his family as have been reported. That, combined with an affection-less childhood, his parents' recent divorce, probably some underlying anger at his mother for supposedly cheating on his father and leaving the family that was transferred to Emma and the other female victims, PLUS the constant images of death and gore he was exposed to through horrorcore, PLUS Emma possibly dumping and belittling him, PLUS other factors none of us will probably ever know - all of this brewed together to create a recipe for disaster at this particular place and time.