1,. He did this was because he was angry about being teased for his poor English skills.
2. He "had been expelled, possibly for behavioral problems."
3. "Howard described Goh as cooperating with authorities although he said he has not been 'particularly remorseful.'"
4. "Goh left behind a string of debts and minor traffic citations in his former home state of Virginia and was evicted from one apartment complex."
This description from the April 3, 2012 Christian Science Monitor is also indicative:
A security guard at the supermarket approached the man because he was acting suspiciously, KGO-TV reported. The man told the guard that he needed to talk to police because he shot people, and the guard called authorities.
"He didn't look like he had a sign of relief on him. He didn't look like he had much of any emotion on his face," said Lisa Resler, who was buying fruit at Safeway with her 4-year-old daughter when she saw the man.Flat affect?
All of these combined lead me to guess that there are serious mental health problems here. Whether anyone recognized them as mental health problems, or he was just assumed to be a troublemaker, remains to be seen.