It seems that Amadia is Astarte, because she is depicted with small horns on every altar (however she doesn't have horns when Fane speaks with her in his visions). Plus, in the first Original Sin game all Source came from First Garden and became corrupted when that place turned into a battlefield between Astarte and the Void Dragon. And Amadia is called the goddess of Source in DOS2.

It also seems that the gods created living beings to be Source farms of sorts. They are born with some amount of Source in them and when they die, their Source is sucked by gods in the Hall of Echoes which serves as a reservoir of sorts. However, Sourcerers can channel Source and thus store much higher amounts of it compared to a normal being. So when you take all Source from them, they become brain-dead, and when you take Source from general population, they don't feel it (but maybe they are denied the afterlife). I also think that the Magisters were able to use the Aetheran to suck all Source from common folks. At least this is how I explain this, the game itself does a poor job in that regard.