I'm not actually sure that the actions of the public went significantly beyond the force necessary to restrain him (into a punishment beating). He was a potentially very dangerous member of an armed gang; it was necessary to subdue him quickly, as he might have had, say, a knife in his waistband. And even if one or two did go a little further than it appears absolutely necessary to us (with the benefit of distance and hindsight) I don't think it's reasonable to criticise - you can't expect untrained people in a dangerous situation to weight things to perfection. And we should remember that the robber made that situation. One in which their actions may have lifelong consequences for the workers they terrified.