Indeed. These days certain things on steemit reminded me so much of the 'ye cannae act as judge and accuser both!' line that I had to see if I could find that old vignette. I'm surprised I was able to.
But, you see, when William Penn and co. gave the blessing to vigilante groups, it was someone in authority giving them the authority to act in specific conditions. There is a huge gulf between that and someone taking the law into their own hands, and dispensing justice according to their own rules, without anyone assigning that task to them. I realize I should have emphasized how the process of justice needs to be viewed as something that is established along the lines of mutual arbitration, whereby all parties agree to respect the process and the final decision reached by the judges. That is crucial to preventing the vigilante-to-vendetta cycle.