The Hero by John Ringo
The human was an expert sniper-and a psychopath. He had never failed in the past when he stalked human prey. But now he is on an enemy planet, and his prey is anything but human. The Darhel are a race with a highly developed empathic sense. Long ago, they learned that they cannot deal death to another intelligent being without being destroyed by the death agonies of their victim. Even though they have been manipulating other species behind the scenes for millennia, including the humans of planet Earth, they cannot bear to kill another being, and depend on other, less sensitive beings to do their dirty work. But now one of the Darhel must kill or be killed. And the fate, not only of his own race, but all of humanity, is riding on his survival. The course of the next thousand years will be determined by whether or not he can learn to fight back. If he cannot, it will be too late...for the entire galaxy.