I do not think mental illness is a factor here.
Granted some of them did, but I doubt it had anything to do with it. Elliot Rodger, Adam Lanza, Eric Harris and Dylan Klebold, and Seung-Hui Cho are injustice collectors. They are resentful at society. It is not just spree killers who are injustice collectors. Many terrorists are injustice collectors like Osama bin Laden, Mohamed Atta, Omar Mateen, and Abu Bakr al-Baghdadi. There are even serial killers who would be considered injustice collectors like Aileen Wuornos, Richard Ramirez, Todd Kohlhepp, and Dean Corll. There are even female murderers who are injustice collectors like Jodi Arias, Betty Broderick, Gertrude Baniszewski, and Lori Drew.
The Dangerous Injustice Collector: Behaviors of Someone Who Never Forgets, Never Forgives, Never Lets Go, and Strikes Back!
http://online.liebertpub.com/doi/pdf/10.1089/vio.2014.1509
Injustice Collector
http://www.psybersquare.com/family/family_injustice.html
Identifying The Next Mass MurdererBefore Its Too Late
https://www.psychologytoday.com/blo...ntifying-the-next-mass-murderer-it-s-too-late
On Wound Collectors
https://www.psychologytoday.com/blog/spycatcher/201509/wound-collectors