I think this song is talking about how humans re becoming more and more selfish. We kill each other and the earth, taking more than our share from it. "Kill kill multiply, till the skies are black and the rivers run dry.." Talking about all the murdering and hate that happens. Multiply, we are overpopulating the earth, and therefore drying the rivers because there isn't enough water to support all these people. And the skies are black from pollution.