By Cheney Orr and Brendan O’Brien GREENWOOD, Ind. (Reuters) -The gunman who killed three people at a shopping mall near Indianapolis over the weekend before an armed bystander shot him dead was a 20-year-old local who had a juvenile record but no criminal history as an adult, the police chief said on Monday. The “Good Samaritan” – who was lawfully carrying a firearm while at the shopping center with his girlfriend – stopped the gunman almost as soon the suspect opened fire on Sunday in the food court of a mall in Greenwood, Indiana, outside of Indianapolis, local officials said. Police identif…