I forget, was it Bec Noir and PM who were killing the Horrorterrors, or was it LE, or was it both? The pretense for Rose and Dave's Green Sun mission implied the former (but since the mission was a trap the pretenses could have been too), and when we see BN and PM in act 6 they're surrounded by sliced-up bits of Horrorterror, but I'm not sure why the Horrorterrors would have sent Dave and Rose on the mission to create the Green Sun just to ensure that BN and PM could kill them for a little while

Lord English was killing Horrorterrors. Dave and Rose were mislead about the nature of the Green Sun mission by Doc Scratch. They thought they would be blowing up the Green Sun in order to de-power Bec Noir.