Chennai, 14/7: A key accused in the murder of BSP Tamil Nadu chief K Armstrong was shot dead here on Sunday after he attempted to assault police personnel and flee from custody, police said.When the accused, K Thiruvengadam, was taken by sleuths to a location in north Chennai as part of investigation to unearth weapons concealed in a spot, he assaulted a policeman and tried to escape and he was shot at by a police officer.