And then there was Jim Jones, and then there was David Koresh, and then there have been other assorted loons that are held up as trophies by the cults they adopted and many died from following them.
Inept loser, unsuccessful in everything, inspired by nothing, drifting through life wondering why life was so hard. Judged by society, and found lacking, seeks others that are NOT superior to his own self-image and reflections of it from others. Looking for the Magic Answer, he discovers an entire life plan, authored by Mohamed, which gives him the structure he was lacking, and "acceptance".
Since it is at least partially a fantasy, he has to "act out" his new religion. He changes "costumes", trading the "slacker" dress (emblematic of an outsider) for the "mullah-look", a sign of acceptance from "other" outsiders. This costume comes with an instruction manual that explains everything.
Being white, young and Californian even allows his to fantasize some level of superiority since he "chose" this life, as opposed to simply being born into it.
Now he has to continue to rationalize his choice, even though there are no signs of his seeking structure and inspiration through "conventional" religions. Of all the religions he has to choose form, he chooses to join the "cult" of anti-religion. While it seems to sound unfair to character Islam as an "anti-religion" consider that Islam is today probably the sole religion that still advocates death, submission or conversion. Even the truly fringe religions don't actively advocate actions "against" non-adherents. Moonies, Scientologists, Anglo-Buddists, and even Satanism don't go that far. Islam stands alone in that regard.
Notice that most American muslims (black, white or brown)choose Islam primarily as a protest, or rejection of something. Islam will continue to be problematic (overtly or covertly), until the Islamic theology advances about 1,000 years and allows acceptance and peacefully coexist, as most other "conventional" and "unconventional" religions do. This guy was primo "cult material", before he ever bought the Koran. That seems undeniable from the profile in the article.
The act of killing to gain special access to Heaven, is more closely aligned with the ancient sacrificial cults, than most any modern "religion".