LONDON — As the shock of the brutal murder of a young member of Parliament began to subside on Friday, there was a growing sense in Britain that something ominous had been unleashed in the country. The increasingly ugly anti-immigrant tone to the campaign over next week’s referendum on Britain’s membership in the European Union, coupled with the violence of English fans at the European soccer championships and now the killing of a lawmaker on the streets of a small town, has left many here feeling that the boundaries of acceptable behavior are breaking down....