Massachusetts firefighters are more than twice as likely to kill themselves as civilians due to untreated depression and PTSD stemming from repeated exposure to death and destruction, according to a new report on mental illness and suicide among first responders.The study by the Boston-based Ruderman Family Foundation, which advocates for people with disabilities, estimates that the suicide rate for Bay State firefighters is 20 per 100,000 people, compared to 9 per 100,000 for civilians.