The Antidote - Brotherly Bond, #6

FBI agent Thomas Mason is closing in on his wife's killer, and he's torn between justice and vengeance. The killer deserves to die for what they did, but Mason would have to betray everything he's fought for his entire life. Teamed up with his old special forces squad mate, Jude Scott, they're going to end this war one way or another. Scott is trying to keep the woman he loves from dying, but her only chance is on her father's private island, where he's working on a cure.
Someone has been killing clients of the company Scott runs security for, and the island seems like the safest place to save her. Unless danger follows them to the island.