The Silent Vigilante by Warren Pearlman 

Frank Young is a man who keeps to himself, but after he loses family in two separate accidents and with the police not being able to find the people that caused the accidents, Frank takes matters into his own hands. After entering the military, Frank meets a weapons dealer, buys some untraceable revolvers and a silencer, and becomes a vigilante. He knows he’s violating the law, but even after becoming a policeman, he still continues the shootings. Along the way, he meets some people that need more help than he does and assists them in getting their lives back together. To his surprise, Frank finds out that a lot of people support what he’s doing and the path he has taken.