Stirling Hunt is an agent with MI6.
To be more accurate, he is "a deniable MI6 black-ops operative who takes his orders from handlers in a special operations division of the Secret Intelligence Service."
His full history, explaining a whole lot more about who he was and why he became such a determined - actually obsessed - terrorist-killing machine, comes in the prequel adventure that was released halfway through the four full-length adventures (as of this writing). In the first two adventures, his motivation was strong but nothing like it was described in that prequel. For that reason, most of what follows here is from that description.
"Hunt was a gifted university student, Olympic-level athlete, and he was in love. Until it was taken from him. After proposing to his beautiful American fiancée, she is taken away from him by a deadly terrorist attack." Elsewhere it is mentioned that "Hunt had been in the first year of his Doctorate, reading Economics at the University of Cape Town, South Africa. He stayed on for postgraduate studies, because he was trying to qualify for the Olympics as a rower, and this was his final shot before the real world kicked in and the obligations started."
It was then that he met, dated, and fell in love with Kelly Armstrong, an American finishing her schooling at the same college. That was before she took a quick trip to the American embassy in Tanzania to sign up for a stint with the Peace Corps and was one of the many victims of the Al Qaeda attack there in 1998.
That loss devastated Hunt, at least until he realized "that he had nothing left to live for, and that the love of his life deserved more than the violent death she'd gotten. Now he only had something to kill for."
What would follow would be a fair number of years training for what would eventually become his life with MI6.