I really feel for the children/teens that might attended, when I was about 10 it was one of my worst fears that something like this happens at some point when "the world" would turn against us. I think the perpetrator might be someone involved with that congo, only time will tell.
Another sad thing aside from the "blood dylemma", possible happening now, is the upcoming propaganda derived from this, it's hard to think of the JWs as one any other group like or religion that may suffer from this kind of acts when you are in.