I believe he was telling the truth about his FTO kicking the suspect.
He was not entirely stable to begin with so when he felt he lost everything, then he felt he had nothing else to lose so he choose to make a statement by killing folks.
That's one reason that having a strong family and good friends can help prevent this sort of thing. Apparently, he has neither.
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“The single biggest problem in communication is the illusion that it has taken place”. George Bernard Shaw