This question was raised once before on this forum and some people said that the Jehovah's Witnesses they knew would definitely not kill if the organization ordered them to.
I disagree with that.
I believe that if they were instructed to do so, most (not all, of course) would gladly kill and believe that they were doing a good thing, a holy thing. They would kill gleefully, willingly. After all, these are the same people who will watch their children die in the name of Jehovah so murdering in the name of this god may cause them some slight consternation, but they would do it all the same.