Violently? I think this assumes there will be some sort of justice system to decide who is criminal and has the power to "dispose" of them. I've heard that the term democracy was used as a derogatory term synonymous with mob rule or anarchy. So yes, I think mob rule should be how we handle justice. If you decide that a person is so dangerous that they need to be disposed of then you just have to figure out how to make that happen I guess.
