Not sure if I was clear about lynch mobs and vigilantes, Bones; I abhor that kind of behaviour. I pretty much run away fast from any kind of group mentality.
So, are you asking: where do we find the healthy balance between the law at one end and lynch mobs at the other?
I'm not sure we can, as an idea, a theory, a thing to be written down and adhered to. But for actions regarding particular situations, I guess we have to individually ask where we stand, what our morals are, what our responsibilities are (to ourselves and others) and act within those thoughts.
The healthy balance is where you or I see someone being cruel and we stop them, or attempt to. The motive is rescue, not persecution (how many lynch mobs are about rescue?).
On the other hand, if I believe that drug-dealers are the scum of the earth and that I'm saving unknown 'good' people by killing them all, I deserve to be locked up - because that's persecution.
Well, that's what I think at the moment. The trouble is, my thinking could change.