There have been over 600,000 applications for asylum seekers into the EU, there is no way to check and see if they are truly fleeing the conflict or if they happen to belong to IS. In the UK we've already seen the government's mishandling of the immigrant problem and that mishandling has led to the rise of UKIP.
When you have a any large group of people who don't share the same values, share the same religion, share the same respect to others, etc as those around them there will be conflict, add to that the inability to check the backgrounds of said people and you're in trouble. All it will take to set off a powder keg of trouble is for ONE person in that group to commit a act of terrorism or another grooming case. Then even those who are trying to be peaceful who are tolerant of others will find themselves in the middle. Then it starts to become a self fulfilling prophecy, those who feel oppressed with fight back, only to be pointed at as why they need to be oppressed even more, the the circle starts.