Aren't police officers supposed to be above all that, with training that should produce a more restrained reaction to violence against them?

After all, the suspect is lying face down and, even though he isn't handcuffed, he appears to be compliant at the moment the officer kicks his head and then stamps on it.

But, although it looks awful, it's difficult to assume anything from those few seconds of a bystander's mobile phone video.

Retired police firearms officer Kevin Hurley told me: "Yes, it looks bad, but we don't know exactly what went on before and we don't know the officer's mindset.

"If a suspect's not complying, the mindset you're going to adopt potentially, especially the firearms officer in an airport, is: is this a terrorism related incident?

"Which is therefore going to mean you're going to take an approach where you want to neutralise a suspect, so they cannot cause more of a problem.

"He seemed to be down with one arm out, but you can't see the other one. It is possible that within this mindset, if the officer decides to make sure the suspect is truly down, he would stamp on his head."

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Did this officer fear a terror attack? He and his colleagues at Manchester airport were responding to reports of an assault, not a terror threat. Four suspects were later arrested for assault and affray, not terror.

Police firearms officers are authorised to use lethal force to protect the lives of others and themselves. But they must justify it. They are not above the law.

Firearms officers on counter terror operations are trained to fire a head shot. I've heard one CT officer talk about aiming for the mandibular nerve, which he described as the 'on-off switch' for the brain, to stop a suspect throwing the switch to a bomb.

Did that scenario play any part in the officer's thought processes? It's difficult to see why it would.

Mr Hurley, a former Met and regional crime squad officer who was also a military firearms instructor and carried a gun as a police adviser in Iraq, said: "Police are allowed to use whatever force they feel reasonable. The officer will have to explain his thinking at the time.

"When he's judged, as he will be now, was it reasonable in the circumstances? If he perceived it as an active terrorist incident and he can explain that, then quite honestly kicking and stamping on his head is not unreasonable."

But maybe the officer, thought to be one of those who was injured, just lost it.