Suggesting that he killed 6 people because of his (possible) mental illness also perpetuates stereotypes surrounding mental health. People with mental illnesses are more likely to victims of crime, not perpetrators.
If you label a killer as "mentally ill" and fail to address any other factors involved, the killer is no longer held responsible for his actions. He is provided with a perfect scapegoat, and it propagates the idea that mentally ill people are dangerous.
Also, out of all the articles I read about this, plenty of them were quick to mention mental illness, yet few of them even touched on that this kid felt so entitled to women that he killed people.
I don't know about you guys, but this is quote doesn't sound unfamiliar to me at all. I hear boys whining about girls not giving them the time of day, even though they're a 'nice guy,' and how girls 'only go for douche bags.' This kind of thinking is rampant, and its dangerous. I mean, just last month a girl got stabbed for turning down a kid to prom.
I wonder how many of these incidents need to happen before people realize that something is wrong.