He appears to have been radicalized. This other source shows his message that he posted to other people before the attack.
"Peace be upon the Messenger of God. May his peace, mercy, and blessings be upon you," he wrote in Arabic.
"Brothers please forgive me for any time I have wronged you, I am going in the path of jihad tonight for the sake of Allah Azzawajal," he then continued in English.
"I am a soldier of the Mujahideen of Al-Qaeda and take responsibility for the actions …
"If you have any illegal or jihadi things online or in real life please make sure to hide them."