The Law of Quantum

During a military experiment, a time traveller who died six hundred years ago re-incarnates. After years of failing to find an explanation for the incident, the British counter-intelligence sends him from the Kuiper Belt to the distant Earth. As an expert in the 20th-21st Century, he has to assist in the capture of a desert agent. But on Earth, team members are not the least interested in the desert. Soon the time traveller must decide: he will kill, or he will be killed. He is forced to take his fate into his own hands; together with his 9mm. But it is hard for him since, in addition to his severe PTSD, he appreciates everything more whose calibre is at least .500 Magnum. But there is no time for emotion in the whirlwind of events, he must keep a cool head. Which is too hard for him to do. Especially when he must meet the memory of his dead loved ones and he knows that changing the past has no effect on what has already happened. Because the future is not written, and the past is not permanent.
A fast-paced story of tomorrow that began fifty years ago. Robots, augmented intelligence, holography, gene modification, and the law of quantum; which we know nothing about but are actively using. Live and let live. An idea that consists of two parts. But you can choose only one of the two. How do you decide?