I believe that he was probably bullied to an extreme, and may have fantasized or even entertained thoughts of murder. These types of ideas are not uncommon in human beings, especially among troubled teens.
Where I take contention is the part where he states that the only reason he did not commit murder was due to lack of being able to access an "assault rifle." Now if we even dismiss the actual definitions of "assault rifle," he could have just as successfully committed mass murder, with or without firearms.
Instead, he chose to jump the Shark and land right in the political anti-gun narrative being pushed today. The "assault rifle" comment killed his credibility in my eyes, and I also feel that it tarnishes his actual experiences from which I have no doubt was traumatic and had great value on its own merit as a learning experience for others. Instead, in the end, I come away with a feeling of skepticism of his motive and detracts from the very real problems and struggles that certain kids endure in school and in life.