"O, wad some Power the giftie gie us /

To see oursels as others see us! /

It wad frae monie a blunder free us, /

An' foolish notion."

– Robert Burns

The US is always collectively amazed, on those rare occasions when it has cause to glimpse at how it is perceived by its less friendly critics abroad. The most egregious example, of course, was 9/11, when even the brutal enormity of the attack against America was not quite enough to still the hateful tongues of people crass enough to insist that the US had got what was coming to it. The citizens of the US have an absolute right to go about their business without being slaughtered. Of course they do. Which is why the world is aghast that this right does not extend as far as Trayvon Martin.

When the unarmed 17-year-old was shot dead by neighbourhood watch volunteer George Zimmerman on 26 February 2012, the killer wasn't even arrested for 44 days, having said that he fired in self-defence. Self-defence? He'd already called the police, telling the operator that Martin was acting suspiciously – "up to no good, on drugs or something". Zimmerman had been told by the operator not to follow the teenager. But nevertheless he found himself and his gun right next to Martin, provoking a struggle. What kind of self-defence is this, when you decide that someone is trouble, and that you're going to stalk him, safe in the knowledge that if things get out of hand … well, you're armed? Yet a jury decided that going out armed, looking for a particular person to defend yourself against, is still self-defence, and on 13 July Zimmerman was acquitted of murder.

Only protest from the public ensured that Zimmerman was tried for killing Martin at all. Only protest from the public has ensured that this killing has been seen through the prism of race. Yet to an outsider, it is obvious that Martin died because he was black, and that Zimmerman walked free after killing him for the same reason.

The jurors say that they acted in strict accordance with the law of the land. They probably did. The law of the land in the US was formulated so that settlers could carry guns in self-defence against their enemies – Native Americans. Later, similar rights over the lives and deaths of slaves pertained. All that is so deeply embedded in the US collective psyche that it's easier to forget that it's there than remember it.

Even though equal civil rights for black Americans are still so new, their achievement still so clear in living memory, the US just can't see what the rest of the world sees – that inequality so entrenched in the history of a state doesn't disappear in matter of decades; on the contrary, the baleful fruits of generations of inequality can be used to justify the very prejudice that promoted the inequality in the first place.

Not that the UK has room to be too superior. British people went off to win the west, and having won it, imported slaves to make it pay. Later, we invited Afro-Caribbean men and women to come and work in Britain, at the jobs that didn't pay enough to attract the incumbent population. Our own history of racism may not have been formalised in a written constitution. But Britain is just like the US in its reluctance to admit that the casual, widespread racism of the past has far-reaching consequences that give succour to those who wish to be racists still. Our own Trayvon Martin is Stephen Lawrence. The awful depths of the hostility of the police to the idea of prosecuting his racist killers is still being revealed, 20 years on, as we learn how undercover officers gathered intelligence into the Lawrence family as they campaigned for justice for their son. Modern states that are worthy of the name are meant to protect their citizens from violence, protecting all of us equally, under the law. In the wake of 9/11, the US and Britain were the most active nations in the world in the quest to take up arms in the cause of spreading liberal democracy. Why neither nation is quite able to see why the targets of this largesse don't quite trust them, when both of us are still demonstrably unable to spread liberal democracy with impunity even among our own citizens, is quite the little mystery.

It's a little-acknowledged fact, yet an unanswerable one, that states exist in great part to maintain a monopoly on violence, either through the activities of their armed forces or via the upholding of the law. The really disturbing thing about cases such as Martin's and Lawrence's is that they reveal how cavalierly states abuse this responsibility. The disconnect in the US can be seen more plainly than in Britain, because the US, as land colonised in recent history, maintains vigilantism as an integral part of its identity so avidly. That's what's at the root of its liberal gun laws – that's what killed Trayvon Martin.

This is one of those moments when the US – and its great ally, the UK – would do well to take a long look at itself. Zimmerman's right to kill in "self-defence" does not contrast well with Edward Snowden's fear of retribution. By exposing the fact that the emails of the citizens of the land of the free (and ours here in Britain) could be plucked from the internet at the state's leisure, wasn't Snowden too defending himself, his fellow citizens, and the idea of the US and of liberal democracy? But no reluctance to arrest Snowden is evident.

We are told that this is all for our own protection – the fight against terrorism is an important part of the state's protection of its monopoly on violence. Yet, not for the first time in these troubled years since 9/11, one wonders how the US can have pretensions to being the world's policeman, when it doesn't even police its own citizens with impartiality. And one wonders how the US can believe that part of its purpose is to be a beacon of democracy and freedom throughout the world, when it clearly believes that it should be able to spy on the private lives of the world's citizens with impunity.

If the Martin case were "just" about racism, then that would be grotesque and awful enough. But it's even more basic than that. It's about the fragility of freedom, and how imperative it is that one person's freedom, like one community's, and one country's, cannot be pursued at the expense of another's. Zimmerman's freedom to get on with his own life has been won at the cost of another man's annihilation. The disregard of the idea that all US citizens have an equal right to freedom and protection could not be made more painfully obvious than this. A monopoly on violence is a terrifying monopoly to hold. It should quite definitely not be shared so casually with self-appointed men from the neighbourhood watch.