They took more than three days to stream by; the lost and the damned burdened by trepidation. They were going home, but there was no joy to it. So silent you could hear the slap of bare feet on the road. All that blood on their hands.

Old people died, babies were born. In the evenings the roadsides were stripped to feed forests of cooking fires, smoke choking the air, and the travellers wrapped themselves in blue plastic sheets and slept side by side; miles of rolled-up exhaustion.

It was November 1996 when I walked with those refugees from the country then called Zaire back into their own nation, Rwanda, in central Africa. There was no pleasure in accompanying them. Many of these people were genocidaires - the perpetrators of a mass killing that still confounds the imagination.

They were coming home because they had no other place to go. They had lived in vast camps across the border in Zaire for more than two years, sustained by aid organisations and an ocean of donations from confused good hearts and guilt-ridden Western governments that had done nothing to stop the genocide.

The last gasp for the refugees had been a vast lava plain called Mugunga a few kilometres from the border city of Goma where they had been trapped by the beginning of a new war - one that would transform corrupt Zaire into the poorly named Democratic Republic of Congo and which would lead to more deaths, millions of them, which continue today. When the war swept by, the refugees had only the road back to the land of their crimes.

The memory of it returned this week with the 20th anniversary of the beginning of Rwanda's genocide.

Nothing ever quite begins and ends so neatly as an anniversary chosen by history, of course.

The Hutu people of Rwanda had been massacring Tutsis since the 1950s when the colonial period began winding to its end. And ever since the genocide of 1994 and the return of the genocidaires the country has struggled to reconcile how victims and perpetrators can continue to live side by side.

Way back in 1933, colonial masters had introduced identity cards separating Rwandans by presumed race. Hutu peasants were the majority, but the minority Tutsi were the pampered and empowered elite.

Identity cards formalised what would become a vicious bigotry, and when the Belgian masters dropped most-favoured status for Tutsis and the Hutu gained power and the killing began in the late 1950s, those cards made choosing victims for slaughter easy.

Following Rwanda's independence in 1962, many Tutsi fled to neighbouring countries. In Uganda, the refugees established the Rwandan Patriotic Front, a guerilla army. It invaded Rwanda in 1990, trying unsuccessfully to overthrow the government. The would-be coup inflamed old Hutu hatred of the Tutsi.

During the early 1990s, open bigotry of the most dangerous kind swept Rwanda. A radio station called Radio Mille Collines and a newspaper broadcast hatred of Tutsis, calling them cockroaches and tall trees. The cockroaches, screamed the radio, had to be exterminated. The tall trees had to be cut down. No one censored the radio or the newspaper.

When forces still unknown engineered hysteria by shooting down the plane of Rwanda's President Juvenal Habyarimana over the Rwandan capital, Kigali, on the evening of April 6, 1994, the ground had been sown for genocide. Lists of death sentences were broadcast, and the radio told Hutus killing was their duty.

The mathematics of what happened is burnt into the world's memory: around 800,000 lives were destroyed in 100 days; 333 every hour, mostly by machete or garden implements. A few fortunate victims were allowed to pay for a bullet.

The Rwandan Patriotic Army had stormed in and stopped most of the slaughter when photographer Steven Siewert and I hitched a ride on a Canadian Air Force transport plane and found ourselves dumped on the mortared tarmac outside Kigali in July of that year, 1994.

Rwanda was a ghost country, filled with dismembered bodies strewn around spooky churches. Afraid of bush dogs roaming dark streets and snarling over things we tried not to think about, we camped at night on the top floor of a looted building, and we spent our days trying to make sense of the unimaginable.

We do not need to revisit the ghastly scenes, though who could not remain haunted by the psychiatric hospital where the staff had been killed or fled, leaving patients locked in cells to starve to death? Sickened, we followed the stream of Hutu who had fled their country and who were dying of cholera and dysentery in Zaire, where they would stay for more than two years.

Before we went down that road - the same one I would travel in the opposite direction in 1996 with the forlorn returnees - I met a Tutsi named Christian who had survived the slaughter.

''How could this happen?'' I asked him.

''It was simple,'' he said. ''We were all the same, we spoke the same language. But we were separated by identity cards, pieces of cardboard, and told to hate and fear. And here is the result. Infectious madness.''

It was at the most extreme end of the definition of bigotry, inflamed by history. And the result was all those broken lives of both victims and perpetrators, and a long way back. If there was anything to remember still on this 20th anniversary of infectious madness, it was this: the idea that free speech should sanction bigotry is shameless nonsense.