I've been watching a few videos on him; for those who don't know he's a 22yr old guy who went on a shooting spree back in May 2014 because he felt denied sex by women.
At first, you'd be tempted to just classify him as an entitled individual however his videos show someone who is seriously disturbed. Some people on the internet point out that he may have co-morbid narcissistic personality disorder along with autism and OCD.
The fact is most people with any mental disorder/syndrome never harm anyone but such disorders are overrepresented among mass shooters in America.
As someone rightly pointed out, genetics from Elliot's parents most likely "loaded the gun" while his dysfunctional family and lavishment by his parents "pulled the trigger" that led to this attack