I think Eugenics raises some interesting ethical and theological questions but also poses, like much advanced technology, some very dark possibilities in the wrong hands.
In the 20th and 21st centuries we’ve seen amazing progress in the sciences which study nature at the levels of the gene, the atom, and the molecule. This has also given humankind enormous power to manipulate nature at various levels. This has brought countless benefits which have made our lives much better, but has also led to some developments which are deeply anti-human and anti-life (such as weapons of mass destruction and widespread abortion).
Catholic theologians and philosophical ethicists will enthusiastically back any medical advance which would ease human misery and suffering and cure disease, unless that advance involves harming human life somewhere else (i.e. involuntary organ donation or destroying stem cell embryoes). The same I think will be the case when in the later 21st century, advances in biotechnology and genetic engineering, as well as the possibilities of nanotechnology, would come to the fore.
The danger with tampering with human genes for producing a certain ‘type’ of ideal human is that the ‘type’ of ideal human will mirror an un-Christian concept of what man’s nature is, and will also include a dark view of what human nature is in the eye of the beholder. In the 1920’s and 1930’s, certain groups of human beings were deemed ‘burdens’ on society because of their racial heritage, parent’s background, poverty, or other circumstances. Many of these judgements were based not on sound science but on pseudoscience and ugly prejudices. While the darkest culmination of this was in Nazi Germany, the same occured in other parts of the world, including the U.S.
In the 21st century, science and technology are far more advanced and powerful, which gives those with the means (financial and otherwise) more power to alter human nature. Human nature, as Christianity has understood it, has a physical-biological nature, but also has a spiritual nature which in out terms would could say, has the ability to partake in the transcendant reality which underlies all existence. Christians call this reality ‘God.’ In Christian understanding, anyone baptised, even those who society rejects, have the capacity to take part in this reality at an intimate level. In Secular Humanistic ideaology, particularly that of ‘transhumanism’, this reality does not exist and the only way for humans to transcend their own condition and nature is by artifically altering our own nature; physical, biological and mental, so eventually we have ‘perfection’ in so far as it is possible for a creature. But this goes against Christian understanding, which believes perfection is only completely possible with God’s grace; human nature is fully realised only in God’s light, both in this life and the life beyond death.
Altering human nature then fundamentally at any level solely for the purpose of ‘deifying’ ourselves (as was the case in 19th and early 20th century atheistic ideaologies such as secular utopianism, Spencerism, Social Darwinism and Marxism) without God goes against Christian understanding and would be judged to be morally wrong with certainty by the future Magesterium and Popes and Doctors of the Church in the coming decades and centuries. Yet, any alteration which simply aims at helping humans live better and more healthy lives; i.e. technology which gives and allows life to flourish, so long as social justice is not destroyed, will be seen as a great good. Catholic ‘eugenics’ then should be seen in this light, though a better term might be Catholic bioethics.