No, Darwin didn't advocate genocide, but it can be considered a natural extension of Darwinism. If man is no different than any other animal, then what's the harm of selectively breeding the populace and culling the inferior, as one would do when attempting to breed champion dogs or horses? It just speeds up the process. Apparantly, all of those people were dragging the rest of the population down, and the proponents of the Holocaust believed it was for the good of the human species as a whole.
There are religions predating Confucionism and Buddhism in China. Before that, it used to be mainly shamanism and ancestor worship, which is still a supernatural moral base.
If man is no better than another animal, then most of the moral code falls to pieces. Just follow your instincts. If you want to do it, it's because eons of natural selection resulted in that instinct. Go ahead and do it. If it's beneficial to you, then you'll pass your genes on. If it kills you, then it's better for the species. You wanna have meaningless sex with a bunch of girls? Go ahead, STDs are just another form of natural selection. A bunch of men? Sure, if it wasn't somehow beneficial, the 'gay gene' would have been culled long ago. Human eugenics, infanticide of children with birth defects, and euthenasia of the handicapped all become morally acceptable once you believe that the human soul is just another outgrowth of natural selection and that man is really isn't any more relevant than a stalk of corn.