I can't keep up with this thread! It's generating more heat than light at this point.
Do immigrants commit more crimes statistically? Do Muslims commit crimes against the person at a higher rate than the rest of the population? It seems to be implied in some of the comments, but I have never read this anywhere. On the other hand, I am sure that I have read that immigrants do commit crimes at a higher rate than the general populations. If this is true, I would guess it is because immigrants tend to skew young and male, and young men are statistically the most likely people to be criminal.
I think, concerns about terrorism, mask an uneasiness about cultural change. A worry that as more immigrants come and immigrant families have higher birth rates our society will change in ways that are uncomfortable for the natives. Islam is a muscular, proselytising religion. Some of the beliefs, values and behaviours can be very challenging for both secular Europeans and more traditional Christian Europeans. I think this is at the base of a lot of the unease, while it is more social acceptable to expressive this unease as a fear of terrorism. We can all agree that ISIS is bad and killing and torturing people is wrong. Admitting that you don't want a Saudi funded mosque with a minaret high than the old cathedral, or that interacting with another mother at a soft play area with her face covered but for her eyes is frustrating is much, much harder.