Before Sal became a merc/bounty hunter, he was actually in the police force. He was a marvelous creation, A brain inside an indestructable body. With the nanites crawling, repairing him constantly from damages he was almost immortal.
He was the ultimate weapon against crime until one unforgiving day, he discovered that crime was even in the police force. As the scandal had been lifted to the media, Sal left never to be seen as police again.