This whole episode is outside of my usual remit. It is a criminal case in the US. However, is it a telling and symbolic case which shows up a number of features of contemporary society -the same in the US as in the UK.
Firstly; why has this particular case garnered such a massive level of media attention? In fact the US police kill hundreds of people every year – and I have no doubt that in many cases this is a case of ‘excessive force’. The reason is the existence of the video of George Floyd being killed. The video is pretty unambiguous. There is no opportunity to point to, for example, a reasonable suspicion that he might have been about to draw a weapon. Mr Floyd was handcuffed and on the ground surrounded by police officers. There is an organised lobby group (nothing wrong with that of course) with an agenda to raise the question of black people being unfairly abused/targeted by the police – the video suited their purposes very well. If we simply had the facts of the case and no video then this case would not have achieved maybe 1% of the coverage it got. The medium is the message.
Secondly; it is clear that George Floyd was not a productive member of society. He was a petty criminal and a social nuisance. The hagiography of George Floyd is a strange phenomenon. It is a good example of the grip which the victim ideology has on contemporary social discourse. To be a victim is to be a saint. The treatment of George Floyd by the media and by much of the political establishment exemplifies this. At best his criminal past – and apparent criminal actions at the time of his death  – are simply forgotten. At worst there are conscious and fanciful attempts to rewrite history. Floyd was a misunderstood individual who had (victim again) problems with opioids (reference the opioid scandal here) who was just turning his life around.
Thirdly; the criminal justice system in its operation shows that we (in this case the US) are a stupid and primitive society. Why did Derek Chauvin not get off George Floyd’s neck? We have seen the video(s). Shot by a large group of agitated bystanders encircling Derek Chauvin and shouting at him what to do. The video is played endlessly – but we don’t see what may have had a significant impact on how events played out: the crowd itself, surrounding the officers and shouting at them. The “observers” are of course players in the total scene. The defence tried to make something of the possible hostility of the crowd I think – but they don’t have to have been hostile. It was probably enough that they were telling Chauvin what to do. My guess (a hypothesis given that I don’t have access to detailed information) is that Chauvin got into a kind of mental reaction where it was a battle of wills between him and the mob. This would explain why he didn’t even get off Floyd’s neck even after he was clearly dead. My guess is that Chauvin would probably profile for Asperger’s – or somewhere near it – if he was examined by a psychiatrist. He didn’t have the mental and emotional capacity to process the complexity of the extremely challenging situation – a large non-compliant threatening suspect, a large crowd shouting at him, and a lack of support from his colleagues.
On the point of his colleagues my guess is that the reason they stood around rather than assist with the arrest is again because of the mob. No one would want to kneel on the ground and turn their back on a mob in such a situation.
In essence – I would hazard an explanation that this was a tragic situation resulting from stupid actions of the crowd (who individually maybe all acted with some degree of good intentions) and a lack of maturity and depth on the part of the police officers – especially Chauvin. And, while it doesn’t “justify” what happened – had Floyd not been on drugs (apparently), not (allegedly) trying it on in the shop  and had he complied with initial police instructions then these too are reasons why this would not have happened.
22.5 years is nothing to do with “justice”. It just reflects a number of non-rational trends in modern society.