Nemesis

Evan Smoak is a former government assassin, once known as Orphan X. He has stayed alive for years by keeping his circle very small and following a strict personal code he calls The Ten Commandments.

His closest friend is Tommy Stojack, a gifted gunsmith who built much of Evan's weapons and combat gear. When Evan believes Tommy has crossed one of his firmest lines, the two fall out, and the argument turns into open war. The rules Evan lives by collapse, and the results are deadly.

Evan sets out to find the only friend he has and confront him. Tommy has left town to keep a promise to help the son of a friend who has died, and he is doing everything he can to save the young man. Evan arrives intent on revenge, but a still more dangerous threat is closing in on them all. The question is whether any of them will survive.

Nemesis is part of Gregg Hurwitz's Orphan X series of thrillers, which follows Evan Smoak.