Among the horrific images from the Virginia Tech massacre, two stand out. The first is of Cho Seung Hui, the gunman posing for his own video epitaph, pausing between killing sprees in order to make his delusional video and post it to a television station, before going out in a blaze of resentment and self-obsession.

The other image can only be imagined. It is of Liviu Librescu, the 76-year-old maths professor who held shut the door of his classroom while his pupils scrambled to safety, and was then shot dead .

Cho intended his multimedia “manifesto” to be profound: a typed 1,800-word diatribe, 28 video clips and 43 digital photos. Clearly, he had thought long and hard about his exit, his insane bid for celebrity, his 15 minutes of infamy.

Librescu, in a separate moral universe, seems to have acted on instinct. Perhaps he simply did what many other teachers would have done, protecting the students in his care. In his eighth decade, he may have calculated that his life was worth risking for others whose adult lives were just beginning.

But perhaps Librescu did what he did because the things he had witnessed in a long life left him in no doubt about how to act. Librescu’s life began as it ended, in a miasma of horrific violence. A Romanian Jew, as a child he was interned in a labour camp in Moldova, deported to a Nazi ghetto and narrowly escaped shipment to the death camps. His engineering career in Romania was ruined by his refusal to swear allegiance to the Communist regime.

Librescu knew a fair bit about standing up to violence and intimidation, which is why, I suspect, he reacted as he did when the gunfire grew nearer down the corridors of Virginia Tech. He was responding to the immediate situation but also, perhaps, to his own history.

A natural reaction to a story like this is to wonder whether one would have behaved in the same way, or whether one would instead have obeyed the impulse for self-preservation. Some people are lucky enough never to have to make such choices; some, like Librescu, make them many times over.

When I was Paris correspondent for this paper, I found myself wondering the same thing whenever I saw one of those formal little plaques nailed to a city wall, commemorating another fallen Resistance fighter. Would I have risked life, family, future, like the courageous few under Nazi occupation, or joined the great majority of French in sullen silence and fearful acquiescence? Answering that question was made no easier by the discovery that some of those Resistance plaques are bogus, part of an organised public relations campaign by Charles de Gaulle to restore France’s self-respect after a shattering war.

“Unhappy the land that is in need of heroes,” thought Bertolt Brecht.

Yet every land needs heroes, particularly in moments of crisis. And if it cannot make them, it may make them up. When Pat Tillman, an American football star who had enlisted in the US Army, was killed in Afghanistan in 2004, he was posthumously decorated, promoted and hailed by the Pentagon as a patriotic American hero who had died in combat with the enemy. Only later did it emerge that Tillman had been killed in a “friendly fire” incident, and apparently harboured deep reservations about the war.

We tend to see wars, disasters and tragedies like the killings at Virginia Tech in stark black and white, as a backdrop for heroism or villainy: Mohammed Atta, the Trade Centre bomber, versus Todd Beamer, declaring “Let’s roll” as he leads a doomed assault on the hijackers of Flight 93; the resistance fighter and the quisling; the ranting gunman and the quiet professor.

But history shows that “the right thing to do” is seldom clear at the time. In moments of terror and confusion, the moral compass tends to spin. Only in retrospect does the correct path seem obvious, even to the person who has set out on it. Rosa Parks, for example, the black woman who ignited the civil rights movement in the American South, could never quite explain why she had refused to give up her seat to a white man on a bus in Montgomery, Alabama. It just seemed right at the time, she said later.

Romain Rolland, the French writer, once observed: “A hero is the one who does what he can. The others do not.” The blood-drenched history of the last century demonstrates that a failure to do whatever one can leaves an open door to tyranny and murder. Though he was a mathematics professor, no one in Virginia Tech knew that history more intimately than Liviu Librescu.

He knew where he came from, who he was and what to do. Cho, by contrast, had no idea who he was. His “manifesto” is just a spew of grievances, against rich kids and religion. His poses are drawn from the iconography of gangsta rap and cheap video games. This was a life without context or meaning. He went to a great deal of trouble to memorialise himself, but achieved only banality.

Librescu was not fighting for a cause, or standing up for his rights; he sought no celebrity; he did not have as much time to reflect on what was happening as Todd Beamer, or the ordinary people who pulled fellow passengers to safety from the London Tube on July 7, 2005. He just did what he could, and he probably did not give it a second thought.

Cho wanted to leave an indelible image, but the one I cannot shake from my mind, even though it is only imaginary, is that of an old man at the end of a meaningful life, slamming the door on evil.

Times Recommends

• Police beg us for food. What hope is there?
• Moral darkness that engulfed Baby P
• More to Advent than cheap chocolate