The tricky thing here is that will any following prosecution put off others from doing the same (restraining, not killing the person running away)?
It was unfortunate he died, although he could have said 'fair enough' and not struggled. I have restrained struggling people, and it is oh so easy to get in to a position where harm could occur quite unwittingly. If someone doesn't struggle though, it's easier and more comfortable for everyone involved.
But back to the start though, what if someone is seen attacking a woman in the street, and 8-10 well built blokes think, 'let's detain him'. Should this case go further and action taken against the people who intervened because of the death, would people think twice about getting involved? Imagine if you were one of them, took the decision to not restrain them, and they raped someone the next day.
It is unfortunate he died how he did, but his fate was in his hands. So not justice in the strictest sense, but an outcome which one person could have avoided if he had realised the game was up. After all, it wasn't like they beat him to death and hung him from a tree.