In the case of Darwin's morals, the answer is quite simple - it is part and parcel of his theory (which BTW is philosophy not science). In fact it is at the heart of his theory. There is much in his works promoting eugenics:
Yet he might by selection do something not only for the bodily constitution and frame of his offspring, but for their intellectual and moral qualities. Both sexes ought to refrain from marriage if they are in any marked degree inferior in body or mind; but such hopes are Utopian and will never be even partially realised until the laws of inheritance are thoroughly known. Everyone does good service, who aids towards this end.
From:Darwin, Descent of Man, Chapter 21.
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With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health. We civilised men, on the other hand, do our utmost to check the process of elimination; we build asylums for the imbecile, the maimed, and the sick; we institute poor-laws; and our medical men exert their utmost skill to save the life of every one to the last moment. There is reason to believe that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man. It is surprising how soon a want of care, or care wrongly directed, leads to the degeneration of a domestic race; but excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed.
Darwin, "The Descent of Man", Chapter V.
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a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life,
From: "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life", last chapter, last paragraph.
This sounds remarkably like the beginnings of genetic counceling, as opposed to eugenics. Individuals with genetic diseases such as Huntington's disease, Tay-Sachs, hemophilia, cystic fibrosis and sickle-cell anemia probably shouldn't reporduce unless they wish to pass on these diseases.
We civilised men, on the other hand, do our utmost to check the process of elimination...
But this is true, we do have a proliferation of genetic diseases as a result of our compassion. Fortunately, medical research continues, and perhaps one day these horrible afflictions will be eliminated. Until then, I am glad genetic counseling is available as a direct result of studies in natural selection.