Sicarii, Part 3 by Adrienne Wilder

When Ben Corbin’s uncle is murdered, he comes into possession of information about the man responsible for the death of his mother and father. A man his uncle appeared to be hunting. Now Ben wants to finish what his uncle began.
Jacob Moser fell into a cycle of drugs and abuse. His will broken, his life destroyed, he had no hope of survival. But the man who saved him is no angel; rather, the remnants left behind when all the complexities of what makes a person human are stripped away, leaving the darkest parts of a man.

Marcel Serghi lives by the rules of his House. Rules ingrained in him as a child under the most brutal conditions meant to hone him into a perfect killer. And throughout his life, he has made vicious enemies. Now innocent lives have been swept up into a storm of revenge against him.
But Marcel isn’t going to save them.
He’s going to change them.
Because life is a gift.