[@Knight Solaire] Yeah, in my mind I wasn't thinking "living weapons" like Tyranids or Zerg but more like Orks (or technically Kroks) or Cravers from Endless Space. 

To refine the idea, imagine you have this planet of large, sometimes savage sometimes not primitives, around a bronze or iron age of technology and civilization. Another much more advance species see these primitives and thinks "hey we could make some good soldiers out of them". So they did exactly that, using their advance tech, gene modding and weaponry and turned these iron age primitives into cybernetic space age monsters they threw into battle, using them to conquer other species and planets and thus preserving their creations into the mythic memories of the species and empires that still exist. When the evil empire was finally destroyed, their cybernetic ex-tribals were also destroyed as sort of revenge for the terrible and horrible things they did to other planets.

Now in the "modern" age, these cybernetic monster-soldiers have returned except they are no longer out to kill and eat everyone like they once were. What they are doing now is trying to build a society and civilization of their own and to make their own future, because the evil empire basically stole their future and their chance to developing into a more advanced civilization because of forcing them into military servitude for god knows how long. Of course, they still have all of the changes and modifications "gifted" to them which is what puts them on a hair trigger with several made specifically to ensure they [i]couldn't[/i] organize in a way to challenge their masters (ie the foundations of making a functioning independent society.) They still have to look like their original terrifying forms because they can't actually live without cybernetics.

This is their rebellion against their masters; a final "fuck you" to the aliens who stole their future from them and turned them into monsters, to try and prove that they are more than just the mindless slaughterers their masters and everyone they fought thought and still think they are and that there is some part of themselves that is still pure and untouched by everything their masters did to them.