Dan Stone is an agent for the CIA.
Technically he is a contract killer for the Agency which means that they can use him to eliminate the people they deem needing such a procedure but if he should be caught or discovered, he is not officially connected to them in any way so they can, and most definitely would, wash their pristine hands of him and leave him to his fate.
His fate when they first contact him is not so good.
Stone grew up in Brooklyn and was content to stay there, especially after meeting Rita in high school. They married right after graduation and he, interested in cooking for years, got a job helping out in a restaurant. After several years, though, he felt his career had stalled and together they decided a change was in order.
That change would be enlisting into the army where Stone would discover a skill he never knew he had; he was a natural sniper. He was so good that he was sent to a military school just to hone that talent.
After two very involved tours in Iraq, he left the Army and he and Rita scapped up enough to open a small restaurant of their own back home. Hard work paid off and the place was proving successful. Unfortunately that meant attracting the attention of the local mob and when relations did not go well, including an altercation with one of their newest enforcers, an enemy was made.
A firebomb thrown into the place one evening to teach Stone and his wife a lesson resulted in the place being destroyed and Rita, now pregnant, killed.
Stone wanted justice. The legal system was not providing it so he called upon his skills learned in the military to get it himself. He was successful but it also meant that he was very much wanted by the authorities as well as the surviving Family members.
Enter this point a representative from the Agency with an offer. Work for them, usually on near-impossible missions where a sniper was called for, and a new life was possible.
So now Stone does what he does best.