(I'm guessing its generally asylum/refugee immigrants people are complaining about, not those from elsewhere in europe)
Refugees/immigrants/etc should be welcomed (I have sponsored refugees in the US), but violent crime must be prevented if someone runs out of chances. You need some recourse when you run out of options and people who have immigrated refuse to act in good faith. Immigration is a gift and a privilege.
I definitely want immigrants to integrate in our society (and am prepared to do this when I travel into another country). But I would not expect anyone to assimilate - which to my understanding means giving up their previous culture and take on the culture of the host country.