Technically the whole Reaper/Catalyst thingie is a combination of a few classic sci-fi tropes, just Bioware didn't go all explicit on it. It's all about ascension, something that Harbinger tells Shepard repeatedly in ME2 ("We are your salvation", etc), and the fact that races other than your own represent only risk.
For more explanation of the second concept, I recommend a book called The Killing Star, by Pellegrino and Zebrowski, if you can find it.
In theory, then, the Catalyst is a shackled AI, programmed to fulfill the wishes of its creators (organics who ascended by becoming the first Reapers), presumably after fighting a vicious war or wars with synthetics that they created. So rather than risk organics attaining the necessary technological prowess to create a race of machines that decide to wipe out all life (once they reach the inevitable conclusion that permitting something to exist that is not them is an unacceptable risk towards their long-term survival), the Catalyst is set up to harvest advanced civilizations and force their ascension into Reapers.
It's a little clumsy, but I've seen worse concepts. It's one part Pohl's Heechee saga, one part Saberhagen's Berserker saga, a dash of Niven's Known Space books, a smidge of Hamilton's Night's Dawn series, and just a trace of Neon Genesis Evangelion.
Like I said, I don't mind the endings, really. I just want an epilogue that tells me what happened after.
