Vows From Darkness by Nastasha LaBrake

Coulta has only known one life: the life of a slave, forced to carry out the cruel commands of a power-hungry lord with generations of anger raging behind his every action. The only friend and family he has ever had is the servant who raised him like an older sister would. The only things he has ever been able to count on are the orders to kill, and the agony it causes him.

But everything changes when he is ordered to assassinate Phelin’s Crown Prince. For the first time, he has the ability to make a choice, the right choice, and change his life for the better – but with that comes the risk of losing the only friend he’s ever had, and everything he stands to gain. His very choice may spark a civil war.