Emmett Till was a 14-year-old black boy from Chicago who was holidaying with relatives in Mississippi when he made two big mistakes.

He went into a local grocery store patronised and owned by whites; worse, he eyed up the young woman behind the counter. For this "crime" the woman's husband and his brother hunted Emmett down, beat him senseless, gouged out his eyes and then dumped the corpse in the Tallahatchie River.

This being the Deep South in the 1950s, the two men were acquitted of the murder, though both confessed to it later, when they were protected by "double jeopardy" legislation.

The only good thing in this dreadful tale was that the dead boy's mother, back in Chicago, was a strong and determined woman.

She turned the funeral, when the hideously disfigured body was eventually returned to Chicago, into a major national event, one that had enormous resonance in the developing civil rights narrative. Crucially, she insisted that the coffin be left open so that the almost unbelievable brutality of the murder was exposed for all to see.

Bob Dylan wrote a simple, eloquent song about the boy's death some years later, and much more recently Emmylou Harris wrote a tender, heart-breaking song about the murder which she signs with a restrained poignancy. Both songs end on gently upbeat, restorative notes: Dylan sings of a greater place to live, Harris of the hope of a kinder world.

Such hopes now appear to be mere dreams as far as the south of the US in concerned, despite the election – not once but twice – of a black, left-leaning President and despite several generations of civil rights agitation and achievement.

It was believed by many civil rights leaders that the terrible death had been such a stain on the nation's collective conscience that it would lead to significant change. Now we must have second thoughts on that, as a jury has just acquitted George Zimmerman of murdering a black male teenager, Trayvon Martin, in Sanford, Florida. Mr Trayvon had no criminal record.

Mr Zimmerman had been charged with second degree murder. He was a volunteer in the local Neighbourhood Watch and known locally as something of "a wanabee cop". His mindset appears to have been that of an over-zealous vigilante.

The verdict clearly implied that Mr Zimmerman must have acted in self defence, despite the fact that Mr Trayvon was unarmed. Mr Zimmermann had called the real cops when he spotted Mr Trayvon, wearing a hoodie, and looking "suspicious". The police told him to wait for officers to arrive. Mr Zimmerman's response was apparently on the lines of "f****** punks – these assholes always get away". So he took the law into his own hands and shot Mr Trayvon dead.

But then Florida has a so-called "stand your ground" law which essentially allows anyone who considers themselves to be in danger to take any action they deem necessary to protect themselves.

In that context, it seems utterly logical that Mr Zimmerman was acquitted, despite the fact that as the Florida prosecutor Bernie De La Rionda said: "A teenager is dead through no fault of his own – he's dead because another man made an assumption."

In the wider context – that of a nation which thought it had achieved at least some progress in civil rights since Emmett Till's harrowing death, and which is currently in the midst of a deep and painful debate about the casual and irresponsible use of guns – the acquittal of Mr Zimmerman seems appalling. Indeed it looks very like a throwback to the evil times when the Ku Klux Klan held sway in several southern states.

So when will America finally wake up and realise that "stand your ground" laws are indefensible? And that firearms can be easily, even glibly, obtained – and are far too often found in the wrong hands?

Many, perhaps most, Americans have believed for two or three generations that Emmett Till did not die in vain. Now, with Mr Zimmerman walking free, you have to wonder.