From what it sounded like on my first check, it was a military operation and not simply the galactic police rolling up in their black-and-whites. Depending on how the story works out I can think of several reasons to have someone like him appear in such a situation. Military horror makes this seem like more than simply a mass shooting, after all. In the backstory I have for him, he is being sent around to kill off different undesirables around several star systems. It wouldn't be much of a stretch, for instance, for this to be the latest in a string of dead stations soaked in blood. Depending on how militarized the society in which this takes place is, it would therefore not be much of a stretch to task military personnel with putting a stop to it.
In any case, it's all subject to change. Took me less than an hour to come up with the character and his backstory. I'm sure I can tweak it as necessary - as we likely all will, if only to bring our technology and backstory enemies in line. Insurrectionists, rebels, what-have-you. Currently the three examples of sample writing don't exactly read like they're from the same universe, know what I mean?