We are apex predators, and our shit contains the condensed toxins from all of the lower rungs on the food chain. The other extreme would be an animal like a cow, which shits basically smellier grass.
That's basically it. A human being that's only eaten plants has much less devastating poops.
I don't think this last conclusion is true. It's really about harmful bacteria, not "toxins". Even vegetarians have a complex digestive system that can harbour pathogens. Perhaps their faeces are safer to use as manure than those from a meat-eating human, but much closer to that than to a cow.