Melhor palpite: Alan Dean Foster 's Damned trilogia . Do resumo da Wikipedia (ênfase minha, alguns bits removidos):
However, most sentient species in the galaxy have evolved to be incapable of committing violence against other sentients (violence of any sort being most un-civilized, but against another sentient being an unthinkable crime), which leaves a shortage of warriors on both sides.
On a mission to find new resources and allies, a Weave scout ship discovers Earth circa late 20th/early 21st century AD and finds that humans are uniquely suited as allies, in that they have the ability to fight.
After the Weave scouts convince volunteers (mercenaries) from Earth to join the war, the tide turns for the Weave. Throughout the book the humans are greatly feared by the rest of the Weave, because of the human race's violent tendencies (the rest of the galaxy's species lived in harmony amongst themselves before they developed enough to reach out into space).
Isto também parece combinar com o que você diz sobre o humano sequestrado de seu barco no primeiro livro, a julgar por esta visão geral :
Will Dulac was a New Orleans composer who thought the tiny reef off Belize would be the perfect spot to drop anchor and finish his latest symphony in solitude. What he found instead was a group of alien visitors — a scouting party for the Weave, looking. for allies among what they believed to be a uniquely warlike race: Humans.
Essa idéia é usada não apenas em uma única série, mas em tantas outras que ela tem sua própria página . TV Tropes, intitulado "Os seres humanos são guerreiros". Especificamente, veja a segunda versão deste tropo:
The second form of this trope makes humans useful to other aliens. Maybe the 'good' aliens have been fighting a losing war against the 'evil' aliens due to psychological limitations, numbers, or lack of training in the art of war. Maybe they are evenly matched or even winning against the enemy but want someone else to get shot at for a change. By working together with humans, they can actually put their advanced technology (or magic) to use effectively; in turn, humans gain the peacetime benefits of the aliens' advanced technology. Other aliens may be less scrupulous about the relationship, considering us less 'valuable partners' and more 'Battle Thralls'.
Muitos exemplos desta forma do tropo são descritos na página da TV Tropos (rolar um pouco para baixo e clicar em 'Literatura' sob o cabeçalho da segunda forma do tropo), então se o Damned trilogia não é o que você está procurando, você ainda pode ser capaz de encontrá-lo lá.