Oh my goodness. So let me get this straight.
Many years ago, some race was all like "Damn, organics are gonna make robots that will kill everyone in the galaxy. Let's do something to stop that, because apparently we just know everything. Here, let's make our own robots who will kill everything, thus preventing those future organics from pretty much doing what we did."
So then they made the reapers, who were then all like "Whelp, looks like we have to kill our creators because our very existence proves what they wanted to stop." The reapers then proceed to laser everyone, all while I can only imagine the creators were highfiving each other and doing the twist for a job well done.
Then the reapers figured to just let species keep advancing, even though it would probably just be easier to wipe everything out once and for all.
So because some asshat race thought they damn well knew everything, they condemned god knows how many species to die. That's just stupid. GG Bioware, hell of a story.