The danger in JW's is less about their theology and more about devout members inability to believe/think that it is not the one true religion. Anything is possible when people aren't in control, and I have blasted my family for this and given examples of the danger of letting someone else think for you... however...
All that being said, with the JW's I believe it is so big, and there are so many inside that are actually on the fringe in a lot of ways, that they wouldn't do anything radical. Now the hardcores in the middle, possibly, but overall I think anything extreme would be isolated and small.
Although some would call turning down medical treatment "radical".