On a vessel that is engineered to the cubic meter of fuel, and where we obsess about whether we need 11 or 12 children born in a given year to keep the population stable, warfare on board is very, very, very bad. There isn't a lot that can break without leading to a collapse of the engineering systems. And there aren't a lot of people you can kill before you impact the population's ability to keep those engineering systems running.
I could describe 100 ways for things to break down. But that is not the challenge here. The challenge is to make all of this actually work. What if we got to the end of this story, and instead of some sort of murderous computer, or a faceless corporation, or Lovecraftian horror of an alien I presented the scariest foe humanity has ever run up against: the future.
*** YOU HAVE DIED *** *** AND EVERYONE IN THIS STORY HAS DIED ... *** *** ... OF OLD AGE ... *** *** ... AFTER SUCCESSFULLY COMPLETING THE *** *** MISSION :-) ***