The disparity goes back over 100 years. Even in 1914, a city such as Philadelphia had 10X the homicide rate of London. The best book I have read on the subject is Raymond Fosdick's "Crime in America and the Police" https://archive.org/details/crimeinamericapo00fosd I think the reasons he points out in the 1910's still basically are the same reasons the crime rate are higher today.

Edit: also, the best book I have read on the more modern problem of violence and urban decay in northern American cities is "Devil's Night" by Ze'Ev Chafets http://www.amazon.com/Devils-Night-Other-Detroit-Vintage-ebo...