Throughout his career, he would be sharing his food with his police escort (after arrest in 1962), helping warders with their essays (also 1962), and earning the respect of their Robben Island counterparts by speaking to them in Afrikaans, which he studied assiduously.
Six warders were murdered at another prison, this one at Matamoros in Tamaulipas.As in Colombia a decade ago, the weakening of the big syndicates in Mexico has merely resulted in them being replaced by scores of smaller, nimbler outfits, which are harder to detect.
He has heard of crowded cells where inmates sleep in shifts; of warders who "sell" juvenile offenders for sex with other cons; and of guards who smuggle weapons, drugs and alcohol to paramilitary inmate gangs.
Prison life was good, the food delicious, the warders kind.
In any case, the law and reality often differ: despite being forbidden from striking, prison warders walked out in 2007 over a pay dispute.
That did not stop its most notorious inmate, Luiz Fernando da Costa (alias "Fernandinho Beira-Mar"), the city's most powerful drug dealer, from obtaining guns and a full set of keys last week, apparently by bribing warders.
In general, they tend to overestimate the likelihood that a prisoner will re-offend, says Lance Lowry of AFSCME Texas Correctional Employees, a warders' union.
Only when he had made that point clear to the warders, as evening fell, did he walk away.
The fleeing warders of S-21 left behind troves of documents outlining the prison's work, but it was Mr Vann Nath, painting his memories in sombre oils, who showed most vividly what had happened there.
Salaries are often so meagre and paid so late that charities working in prisons sometimes offer food and soap to warders as well as inmates.
Indeed, his warders were among those who came to admire him most.In this section Invictus Would Modi save India or wreck it?
