The necromorphs are not the most evil force at work here, the necros combat the marker, they didnt want isaac getting to the marker in dead space 2, they dont want fresh corpses for the convergance event . . . they are combating the marker
The marker oesnt create the necromorphs, the necromorph pathogen is loose infecting people but only activates when they know a marker is present. They are a sleeper army ready to combat the evils of this alien device
this alien device wants to recreate its original creators by harvesting life. The dead space is necessary to combat the necromorphs, this is why the red marker was more benevolent in the first game . . . it was helpless it wasn't big enough to protect itself from the necros . . . it needed us.
The necros taint corpses so that the markers cannot use them, THEY killed off the dinosaurs so the marker could not feed
The necros arent a flood ripoff, the necros are a ripoff of the halo itself an the marker is the ripoff of the flood
WE HAVE HAD IT BACKWARDS
This is an incomplete theory . . . but think about it, the necromorphs are the less evil force (still pretty evil with infecting babies and all that)
(also to support this theory I will say that for this theory to work the voice other than the marker in martyr is the necromorphs)