That aspect doesn't make sense as it is, because the technology they leave behind helps develop synthetics, too. So in part they create the problem they say they are there to solve.That's worse to me because it would not make sense that they left the Mass Effect relays and technology to further develop Mass Effect technology, then come along and kill everyone for using it.
It makes more sense with the dark energy problem. They need to absorb advanced civilizations in hopes of finding a solution. So they water the field, add fertilizer and hope to speed development along so they can increase their processing power faster than they could if they waited for every species to develop naturally.
The thing that throws a spanner into that is ME2, where for some reason humans are special because of their genetic variety (which is tenuous in itself, but we are led to believe the other species are even worse). Technologically, the Asari would be the better choice.