And interesting you brought up the San Ysidro McDonald's shooting, Motive and irony there to.
The suspect was a security guard and was fired from this position one week before the McDonald's shooting.
It took a while to find or piece together motives, and to agree on what they were.
There are plenty of motives for the current shooting. Mental problems, depression, suicidal impulses, outsider, family history of anti-social behavior.
Take your pick.