There are more precise professional terms used these days, but "paranoid schizophrenia" will do for a lay discussion. This guy had it all: delusions of grandeur, delusions of persecution, a decade of medication he had begun refusing to take when he turned 18, parents/guardians in denial, and a truly remarkable ability to seem normal in casual conversation until the worms began crawling out of his brain. He killed his roommates with a knife while they were sleeping before embarking on his murder mission. They had done nothing whatever to him.
His was, to be sure, a corner case, and "the System" nearly stopped him. He could not purchase firearms in California, for example, or in neighboring Nevada, and so had to drive three states before he found a source willing to sell to him. He was reported to the police by two therapists, a psychologist and a psychiatrist, as a potentially dangerous and violent individual mere hours before he went on his rampage. So close. What is less forgivable is the length of time his condition was hidden, minimized, rationalized off, forgiven. The people who did so regret it bitterly now but the relatives of his victims regret it a lot more.
However, I had pinged you today to give you the link to the very riveting follow-up story in today's DailyMail about the survivors. For them, it will never be over.