We tell three stories about how a human being can respond to barbarity. One is the tragedy of revenge. One offers the hope of forgiveness. And one diverts itself with furious activity in an attempt to forget. But the story of Eric Lomax refuses to conform neatly to such templates.

Lomax, who died on Monday aged 93, was one of thousands of British soldiers taken prisoner by the Japanese in 1942 and forced to build the 418-mile railway to Burma. He was tortured until there was not a patch of unbruised skin between his shoulders and knees; 900 blows in six hours broke arms and ribs. At night he was confined to a cage coated in his own excrement.

Somehow he survived. But back in Scotland, Lomax was tormented for decades by nightmares. Post-traumatic stress led to estrangement from his father and the breakdown of his marriage. The mental scars refused to fade.

Then, on a long train journey, he met a Canadian woman 17 years his junior and began, for the first time, to talk about his wartime experiences. Despite violent mood swings and week-long black silences, she married him. The trauma worsened on retirement, and he spent the Eighties trying to track down his chief torturer and fantasising about revenge.

But some 40 years after the war ended, at the newly-formed Medical Foundation for the Care of Victims of Torture, he heard of a book written by the man. In it, Takashi Nagase told how, after the war, he worked for the Allies to locate thousands of burial sites along the Burma Railway. Ashamed of his wartime actions, Nagase set up a Buddhist temple at the river Kwai to atone. He was still haunted, he wrote, by the brutal torture of one particular prisoner. But the depths of his remorse brought him the feeling he had been forgiven.

As he read, Lomax realised he was the prisoner. He would never forgive. His indignant wife wrote to Nagase and said so. A meeting at the river Kwai was arranged between the two old enemies. Lomax set off, simmering with rage.

Anger, hurt and bitterness is the commonest response to cruelty. It fuels sectarianism in Iran, resistance in Afghanistan and the blood feuds of Syria. Often it does not abate. When the killers of James Bulger were released, eight years after the murder, the dead toddler's mother said: "I never knew I had so much hate in me." Winnie Johnson, the mother of one of the victims of the Moors murderers, could not forgive for almost half a century until she died recently.

Forgiveness is the hardest response: we have to scour recent history for examples. In 1987, Gordon Wilson stunned the world by announcing that he forgave the IRA bombers who killed his daughter at the annual Remembrance Day service in Enniskillen where she died clutching his hand. The world looks on in awe and bewilderment at such huge gestures.

Indeed, we may resent them. When Gee Walker in Liverpool in 2005 declared that she forgave the racist murderers of her 18-year-old son, Anthony, some newspapers actually criticised her and said that murdering scum did not need forgiving. Her reaction, they implied, was somehow unnatural; we should fight hate with hate.

The world has much more fellow feeling with Julie Nicholson, whose daughter Jenny was killed in the 7/7 London bombings the same year. Mrs Nicholson resigned as an Anglican priest to avoid the hypocrisy of having to preach a forgiveness she did not feel. She preferred "getting on with leading a good and creative life and making a difference in the world". Many bereaved families set up foundations to combat the evils that killed their child in an attempt to prove that they did not die in vain.

Eric Lomax was different. He set out almost 50 years after the war to meet Takashi Nagase, his heart filled with utter loathing and hate. But something extraordinary happened. "When we met, Nagase greeted me with a formal bow," he wrote on the website of the Forgiveness Project. "I took his hand and said in Japanese, 'Good morning, Mr Nagase, how are you?' He was trembling and crying, and he said over and over again: 'I am so sorry, so very sorry'. Lomax found himself saying: "We both survived." Forgiveness turned to a friendship of nearly two decades.

Asked what she felt about the two youths who had killed her son, Gee Walker replied: "I'd love to do the motherly thing and sit them down and find out why... what's missing in their lives." Like Eric Lomax, she saw people, not perpetrators, before her. She also instinctively understood what he had learnt the hard way: the cost of not forgiving. "Why live a life sentence?" she asked. "Hate killed my son. Why should I be a victim too?" Forgiveness set her free.

Forgiveness is more a process than an instinct, as Lomax shows in his extraordinary book The Railway Man. "Some time the hating has to stop," he wrote. It took him 50 years to exorcise his demons. Few of us have the strength for that.