I don't think the key takeaway from this tragic case is that members of the public should not challenge wrongdoing where they see it.
As gingerjon calls out, the cyclist was using a shared cycleway used by both pedestrians and cyclists. The victim therefore was not committing any wrongdoing, they were simply going about their business.
This is an unusual case because the territorial nature of the defendant is more typically found amongst motorists. I commute a 50 mile round trip three days a week on the bike, and I'm sorry to say that there is a significant minority of motorists who believe I have no right at all to be on a road with them. To pre-empt any of the usual tropes - I have insurance, I stop at every red light, and I never cycle on the pavement. I'm not perfect (who is?) but I try my best to follow the Highway Code to the letter. The point I'm making is that at least once a week I will cop a volley of abuse - or worse (deliberate dangerous driving) - simply for existing on the road.
This is the same attitude that caused this man's death.