Most media scoops do not arrive in the morning post, having been dispatched in the pause between two bouts of murder.

When NBC News decided to broadcast parts of the package of video and pictures which Cho Seung Hui had chosen to send to it alone, the television network knew that it would be accused of failing to resist a scoop beyond its dreams, and of pursuing sensationalism and its own profit at the expense of national sensitivity and safety.

It was. Victims’ relatives abruptly cancelled television appearances in protest. The police, who saw the material before it was broadcast, later said they were sorry that people who were not used to such images were exposed to them. Others attacked NBC on the classic grounds that this would give Cho the oxygen of publicity (an awkward metaphor, given that he is dead), grant his wish for immortality and prompt copycats.

Yet NBC, which says it broadcast only after fierce internal debate, and tightly limited the choice and repetition of the material, was surely right to go ahead. People’s shock this week is understandable. But that has brought a tendency to exaggerate the distress or danger of the broadcast, and to dismiss the useful conclusions from seeing it — and even the reassurance it gives.

Watching just a few minutes of the rambling manifesto of paranoia answers the question the US has asked itself for three days: why did he do it? Cho was clearly mentally ill, not simply a troubled student in a bad patch, or someone who snapped under sudden strain; on its own, that is reassuring. Nothing was impulsive, from the purchase of the two guns in two months, to the obsessive assembly of pictures and speeches-to-camera in a digital collage. The paranoia, the sexual and religious metaphors, the flailing accusations at rich classmates and Jesus, the conviction that he had a cancer of the mind — these tell us that the quest to “understand what made him do it” is not going to take us far.

That answer might seem brusque. Yet the video shows how different Cho was from his classmates and from the population (despite the mild American accent which showed he had drawn something from the culture). He was not even much like Islamic suicide bombers, although his recording resembles their final messages, with the black terrorist garb and the weapons. But they spell out their jihadist cause with faux-military succinctness; his had the coherence of a bedroom stack of horror comics, ripped and pasted together. Virginia Tech, and his classmates, might also find it reassuring that his sentiments in the video were so well hidden behind his almost complete silence in daily life, even if they leaked into his literature classes, to general alarm. Nick Jeremiah, a graduate student, said of the video: “That’s got to be more than he’s spoken, ever. I thought, ‘Well, he does talk’.” As one university official pointed out, Cho’s room-mate had not felt cause to sound the alarm, nor five others living close by, however clearly the video shows his disturbance. But this also shows how hard it is to anticipate other cases, although many now seem to be itching to demand this feat of foresight of poetry teachers and counsellors.

The accusation that the NBC broadcasts may provoke copycat attacks — the most serious charge against the network — appears to rest on a notion of severe mental illness as contagious, common and predictable. True, someone who is severely disturbed might want to better Cho’s “record” — but that does not mean that if his video were kept off the airwaves that person would not find other provocation. If only.

But given that a small proportion of people do have some severe disturbance, Cho’s case does suggest that there might be more stringent bars to buying guns than merely asking a purchaser, in a standard form about mental health, to tick a box.The NBC clips reminded us about the unpredictability of mental illness. It was right to broadcast them. But for those who found the distress too great, after three days in which the world’s largest media have shrunk themselves to a single subject, one option is simply to switch off.

Times Recommends

• No apology for Germany’s own Nazi victims
• Intricate web of fear and hope tying down deal
• Victims remembered a year after Mumbai attacks