The Euclidian: Alien Hitman. The Euclidian, #4

This prequel to the Euclidian series follows the exploits of Adar, an alien from the Ossuary System, hired as an elite assassin aboard the Euclidian resource ship Andrea. Adar was sent to retrieve, dead or alive, renegade aliens who started a gang war on their home planet and killed a group of law enforcement officers who chased after them. Adar uncovered that they had been stowed away aboard the Earth-bound Andrea and tracked them all the way to Chicago before the trail goes cold.
While waiting for them to turn up, Adar fed his thirst for blood by taking on Chicago's most notorious mob family. Throughout his journey, Adar learned how Earthlings live and love and discovered his own need for love.