Nemesis By Gregg Hurwitz

NO GREATER FRIEND
Evan Smoak is the guy you want on your side. A former government black ops assassin, he'll be there for you in your darkest hour - and he'll rain hell and thunder on those who caused your pain.

NO WORSE ENEMY
A lone wolf operator, Evan can't afford to lose any of the few people he likes and trusts. But when Tommy Stojack, Evan's loyal, long-time gunsmith and armourer, betrays him, it's war.

Because a good assassin keeps his friends close, but his weapons closer . . .