The Hero is a novel by John Ringo

John Ringo

John Ringo is an American science fiction and military fiction author. He has had several New York Times best sellers. His books range from straightforward science fiction to a mix of military and political thrillers...

 and Michael Z. Williamson

Michael Z. Williamson

Michael Z. Williamson is a science fiction and military fiction author. Born in Birkenhead, England, he and his family emigrated to Canada, then the United States in 1978. Williamson frequently utilizes the pen names "Mad Mike", and "Crazy Einar". He is retired from the United States...

, and is part
of the Legacy of the Aldenata

Legacy of the Aldenata

The Legacy of the Aldenata, also known as the Posleen War Series is the fictional universe of one of John Ringo's military science fiction series.- Premise :...

series. It is set in the future after the defeat of the Posleen

Posleen

Posleen are a fictional alien race, created by the Sci-Fi author John Ringo for his Legacy of the Aldenata series of books. The Posleen are a race of genetically engineered reptilian centaurs, supposedly designed by the long lost race - the Aldenata - to be the ultimate warriors...

, and features a Darhel named Tirdal who is the first of his race to be assigned to a military team. The Darhel have a genetically programmed limitation - they cannot kill, or the descend into a type of madness. When things go wrong on a mission, Tirdal must somehow overcome this disadvantage, or lose his life, and possibly the future of his people.