If it turns out that those responsible were refugees it'll be yet more proof of the dangers inherent in admitting large numbers of people into the country when you have little or no real idea who they are, what their motives are or even how mentally ill/scarred they may be (which could of course be through no fault of their own).
It's all very well offering refuge but unless there are safeguards, services, benefits, education, training, jobs etc. put in place we're likely asking for a whole heap of future trouble from people who become radicalised because, rightly or wrongly, they feel badly let down. Even if we do all we reasonably can there'll be those who feel it isn't enough and decide to take revenge in some way and that's aside from any who come here with the sole intention of doing us harm although they're likely to be a very small minority I'd have thought. What the answer to this is God only knows...
As regards the host family, yes it is awful that they're being targeted but it'd be even more awful that those they took in and cared for may have had become involved in something so awful and possibly even endangered them.