Jack Sullivan, the Specialist, is an assassin.
In fact, he is one of the very best in the business and his fees reflect that. As stated in his first chronicled adventure, his price is as high as the risks he took. Nevertheless, he doesn't take just any job that comes along with adequate payment. His personal code of ethics makes him determine for himself whether the target is deserving of elimination and only then will he take an assignment.
Sullivan is a freelance agent, available to many but with a long list of those for whom he would not accept a commission. Foremost on this list is the Mafia and the Soviet Union for it was most probably one of these two groups responsible for the death of his wife. This loss, and the determination to get the ones who did it, spurs him on.
A veteran of two wars, Sullivan was actively recruited by the CIA but he finally decided to remain independent, though some of his missions might have also sanctioned by the Agency. Though he is not a legitimate employee of any government, the international nature of his work certainly qualifies him for inclusion here.
The Specialist series is a brutal one with violence being the entire reason for its existence. There is no subtlety in these pages, but then again, there is no promise of it. Since Sullivan's job is to kill and, taking the fact that he makes sure innocents do not get caught in the carnage, once these innocents are out of the way, the word mercy does not exist. If Sullivan decides the victim is going to die, he will definitely and surely die.
Note: The first book in the series, "A Talent For Revenge", provided the basic suggestions to the Sylvester Stallone/Sharon Stone movie entitled The Specialist on which the author was said to have helped with screenplay.