I would not be surprised he had trouble fitting in. He probably has a history of bullying as well.
I can picture him as someone complains a lot and is very off putting as a result.
Websleuth Radio Interview Tina Meier
http://www.blogtalkradio.com/websleuths/2013/09/30/tricias-true-crime-radio-adult-cyber-bullying
It starts at the 20:40 mark from the September 2013 Websleuths Radio Interview. Tina Meier, Megan Meier's mother, is describing Lori Drew as a very talkative person who complains nonstop about how life is unfair to her. She is described as a very unhappy and bitter person in a unhappy marriage. Drew wanted everyone to know her plight and suffering in life. Meier's description of Drew is consistent with an injustice collector.
Elliot Rodger: An Analysis
http://www.blogtalkradio.com/websleuths/2013/09/30/tricias-true-crime-radio-adult-cyber-bullying
From the article.
Beyond his entitlement, lack of empathy, and sadism, Rodger was also what the FBI report on school shooters labeled an injustice collector. This was described as follows: The student nurses resentment over real or perceived injustices. No matter how much time has passed, the injustice collector will not forget or forgive those wrongs or the people he or she believes are responsible (OToole, 2000, 17). Rodger went through life accumulating a sense of having been wronged by women; he nursed his resentment until it drove him to murder. Such resentment appears to be a product of his narcissism as he was quoted above, How dare they not give him the love and adoration he deserved.
Meier's description describes Stephen Paddock, Sayfullo Saipov, Omar Mateen, Mohamed Lahouaiej Bouhlel, Aaron Alexis, Elliot Rodger, Adam Lanza, Seung-Hui Cho, Eric Harris, Andrew Kehoe, 9/11 Terrorists, and Osama bin Laden. Like Drew, they live a life of extreme resentment.
To further that, in the Investigation Discovery show Web Of Lies, Friend Request, Tina Meier states that Lori Drew did not take it well that they were transferring Megan Meier to a new school. That suggests that would be a tipping point for Drew on top Sarah no longer being friends with Megan. This indicates an injustice collector as they have tipping points on top of ever slightings they have repressed all those years.
I would not be surprised if we see all that in Devin Kelley. That has been the case with Stephen Paddock, Omar Mateen, Seung-Hui Cho, Adam Lanza, Eric Harris, Sayfullo Saipov, Mohamed Lahouaiej Bouhlel, and 9/11 Terrorists. This also applied with Jodi Arias, Michelle Carter, Casey Anthony, Betty Broderick, Yoselyn Ortega, and Lori Drew.