Not enough has been released to tell for sure. Paranoia, in my professional experience, is the most important element to understand in the possible motives of mass shootings. Virtually all mass shooters are paranoid to some pathological degree. Some of them have suspicious personalities but otherwise maintain a connection to reality. Others have paranoid delusional disorder and have irrational and fixed false ideas about a particular theme. The most extreme of those with paranoia have schizophrenia, a condition that may be associated with intense hostility and different degrees of emotional and mental limitation and — particularly important to mass shooting — progressive and humiliating decline and alienation.
What then leads you to believe Cho had schizophrenia?
How he related to his roommate was just too bizarre to be depression. The bizarre content of his plays -- mashing a half-eaten "banana bar" in someone's mouth, the hypersexual, nihilistic (death obsessed) obsessions in the absence of depressive guilt or tearfulness are another clue. The progressive decline of a period of years. Those with schizophrenia, especially in their earliest years, are not readily recognizable as such -- their condition is evolving. But here was someone who, as early as 2005, was carrying himself so strangely that he was a spectacle. The depressed withdraw and disappear. Those who are so peculiar in their manner so as to be inappropriate (taking cell phone pictures of his teacher, speaking inaudibly, pulling a cap low over his eyes) exhibit signs and symptoms more indicative of schizophrenia. He was communicating in a rambling manner reflective of what we appreciate as autistic thinking -- characteristic of schizophrenia. In a similar vein, Mr. Cho's stilted communication in his homicide note (deceitful charlatans -- not the language of a 23-year-old college kid) is also the manner of a schizophrenic's communications, as is his pronounced delay in responding to questions.
What explains that?
The most obvious reason for a college-age man acting with the maturity level of a self-absorbed high school or middle school student is a major mental illness such as schizophrenia, which arrests psychological development from the point of its dominance in a person's life. A person who develops full-blown schizophrenia at 15 or 16, for example, will mature at a far more arrested pace than a person without such an affliction.
Is mass shooting only a matter of paranoia ?
No, because despair contributes to a person's resignation that his lot will not improve. While many mass shooters have depression, many do not. And depression is not what sparks the mass shooting, hopelessness is.
We also are left, as forensic psychiatrists, with having to understand why something happened on that particular day. There may be a clear conflict; in my experience, there is also an unconscious trigger as well. That trigger may be all the more lethal because mass shooters may be less likely to introspectively reflect that they are wrestling with other conflicts as well.
The unconscious trigger, inspiring a sense of life failure and hopelessness, is always elusive and especially so because the killer takes the secret with him when he dies.