Police have released the rantings of  Abdul Razak Ali Artan from his Facebook page.  Reading it, there is no mistaking what lead him to launch the attack on his fellow students:

Key points in his manifesto include:

• Dawla in al-sham is a reference to Daesh, or ISIS. He indicates that the only way to stop the “lone wolf attacks” is to leave ISIS alone — but he does not suggest that ISIS will then stop killing, only that the attacks in America might stop.
• His reference to “Muslims who disapprove” being sleeper cells implies that American Muslims — particularly those who appear to be “moderate” — are engaged in the practice of Taqqiya (lying to infidels in order to further Islam).
• The celebrity scholars who “sold their deen” refers to clerics whom he believes are claiming to represent Islam while selling out their heritage for earthly fame.