Let’s start by stipulating that not every newcomer to America is a violent criminal bringing old ethnic and religious conflicts to the New World; nor is he a predatory opportunist, happy to take advantage of the stable, trusting, and yes, welcoming culture he finds. Every country has its social customs and unwritten rules, and often newcomers misinterpret them as weakness.
When the Catholic Irish arrived in large number from the middle of the 19th century on, they brought with them a deep suspicion of the police, a distrust of the Protestant ascendancy, and a pent-up willingness to work the system to their advantage — cultural survival skills they had learned during nearly a millennium of English occupation of their home country. It took them more than a full century to fully integrate into American society. Swaths of American cities, especially New York, had Irish no-go zones, into which even the Irish cops feared to tread, and even today we remember the names of the psychopath Mad Dog Coll and the suave but violent Owney Madden, chief of the Gopher Gang, who gave the world Mae West, Primo Carnera, the Cotton Club and Bill Clinton.
Similarly, the immigrant Jews from Russia and the Pale went through a short but violent gangster period (Monk Eastman, Lepke, Meyer Lansky, Benny Siegel), culminating, along with newly arrived Italians, as the triggermen for Murder, Inc. And the Sicilian contribution to urban mayhem has been well documented in print, stage, and screen.