Stormblood

Vakov Fukasawa used to be a Reaper: a bio-enhanced soldier fighting for the Harmony, against a brutal invading empire. He's still fighting now, on a different battlefield: taking on stormtech. To make him a perfect soldier, Harmony injected him with the DNA of an extinct alien race, altering his body chemistry and leaving him permanently addicted to adrenaline and aggression. But although they meant to create soldiers, at the same time Harmony created a new drug market that has millions hopelessly addicted to their own body chemistry.

Vakov may have walked away from Harmony, but they still know where to find him, and his former Reaper colleagues are being murdered by someone, or something - and Vakov is appalled to learn his estranged brother is involved. Suddenly it's an investigation he can't turn down . . . but the closer he comes to the truth, the more addicted to stormtech he becomes.
And it's possible the war isn't over, after all . . .