See my post further down the thread
The New York Post said that a note, several pages long, was found on his body. It may shed some light on his motive for the shooting.
He may have believed that he had a traumatic brain injury, which is believed to be caused by repeated blows to the head. It's frequently found in people who play rough contact sports, such as football.
The Wikipedia entry that I found for the specific injury said that it has been found in 99 percent of the brains of deceased NFL players.
The shooter requested, in his note, for his brain to be studied. Wikipedia says the condition caused by the repeated blows to the head can only be properly diagnosed through an autopsy.
I know I will probably get chastised for speculating, but I think that your guess is correct.
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