Because of uncontrolled immigration people with serious criminal records have been allowed to enter Britain and commit crimes.
Under a controlled system of immigration would be immigrants have to undergo criminal record checks ,if they have a criminal record they don't get in, even you can work that out.
A couple of years ago a Latvian migrant who had done time for murder in his own country decided to come to London because there is no longer any need for EU migrants to go through the Visa process he was able to come to London and murder a young girl.
There are plenty more like him.