The Assassin's Legacy by Libby Webber

A love to kill for. A love to die for. And a secret to keep the body count rising. Aeron’s Legacy is to protect the innocent. Luke’s Legacy is to lead his fellow assassins into a new generation. When a traitor tries to kill them both, they must gamble lives they’ve sworn to protect to ensure the survival of their hidden society. But only one of these assassins has the true murderer in their sights while the other gambles an innocent life. As the body count rises around them, Aeron and Luke must determine which lives are worth saving and which deaths they can justify in order to protect each other and stop the Legacy from being destroyed.