There is a possibility, however remote, that the witlesses (or some other cult) will kill infidels during the supposed Armageddon. Christian cults, however, do not generally do this. Rather, they will cut themselves off from association with outsiders. Some will commit suicide, but murder among Christian cults is not likely.
What they might do is suppress freedom of speech among the world. They could use the court system to alter the language, fixing things so people cannot communicate. More practical, they could influence the world by suing people that do not go along with their rules. They could, for instance, threaten and sue people that set up Christmas decorations on the grounds of offending them on religious grounds. In today's "politically correct" non-freedom, this is the most probable course. They could well play that "politcal correctness" card to diminish freedoms of the world to evade the witlesses.
Now, I do think cults do have the capacity to take matters into their own hands. Usually those are Islam cults that will murder those that do not comply. We have seen many examples of that in the Middle East. Christian cults do not mass-murder, but they threaten that God is going to mass murder those who do not obey.
As to what individuals choose to do, that's another matter. Even in Christian cults, individual cult members might take it on themselves to threaten and/or kill those that are close to them unless they turn around. They might threaten to ruin family members' credit rating, or rack up a criminal record, unless the member joins and/or returns to the cult. It is not unheard of for individual witlesses to break into homes and sabotage things, put poison in their food, or steal credit card numbers. However, this is not likely to be the general cult-wide pattern.
