Death Comes But Once by Eric Thomson

Zack Decker wanted only one thing in life: to fight as a Marine Pathfinder. But his temper and his loathing for careerist officers cost him dearly. Forced into early retirement, with a wife who left him long ago, no job, no friends and no future, the former sergeant crawled into a bottle and stayed there, moving from planet to planet, looking for some reason to keep on living. Until, that is, he wandered into a bar owned by an old Marine buddy who offered to help. On the run after killing a corrupt cop, and nowhere to hide, he accepted.

Decker would have done better to head for the hills, or surrender to the militia, however old friends could be persuasive and he soon found himself signing on as security officer of a starship with dubious owners and a dubious crew, doing things for his new employer that tested the old saying 'Once a Marine, always a Marine.' Curiosity was Decker's other major failing and in short order he was on the run, hunted by a shadowy government organisation, and in possession of a secret that could destroy the Commonwealth. It was a secret to die for, and the people on his trail wanted to make sure he did. Along the way, they forgot that he could still fight like one of the Few...