Thirty-one inmates were slain Friday in northern Brazil, some with their hearts and intestines ripped out, during prison killings led by the country’s largest gang, authorities said. The bloodshed comes just days after 60 inmates were killed during rioting at two prisons in a neighboring state and it increases fears that violence could spread, including to the streets of major cities, as gangs vie for influence and territory both inside prisons and in slums where trafficking operations are often based.