This is the problem with extremists. This thing happens now via the Islam but Catholics and Protestants also have a reputation. Every religion where soft or strong hatred is preached will generate these individuals. In all fairness, JW's do not take up literal weapons but verbal and behavioral weapons (shunning, silencing..). If the leaders of the WT would preach a harsher message, I am sure that among the 8 million JW's a few will standup and use violence.
I do not like Islam but refuse to generalize. Of the 800 million or so Muslims, most are normal people with their own traditions and practices, not harming anyone. It is the few (out of 800 million it is still a lot) that have become "executioners" of their god.