[Crazy Jetty]

Wild theory:
Atrocitus spoke a couple times implying that it was the Green Lanterns that destroyed his homeworld. His hatred of the Guardians (Like in the comics) comes by extention of this since they're the creators and ultimately responcible.
He never once mentioned it was the Manhunters.

In the comics, he hates the GLC not because of who they are and what they represent, but because they were created by the Guardians.

So... we all know the GLC would never be complicit and commit genocide because the Guardians told them to do it. (Not unless there was an accident, like Stewert accidentally destroying the one planet).

So my theory is this... The Sinestro Corps are the ones responcible for destroying Atrocitus' home and people, posing as the GLC.