Getting back on tipic... Well, I personally divide it in to categories, generally starting with the easier calls (for me) and ending with the tougher ones. From a completely agnostic perspective, here's what I would say concerning Eugenics itself, without regarding the divine implications.
In the case of people who are useless to themselves and to others, ie vegetables, or people born with such terrible conditions that they would be unable to develop higher brain function before dying a painful death at a young age, I tend to find it morally sound to remove them immediately, as to end their suffering, end the suffering it brings to others, and lighten the load on society. If the only positive points are extremely unlikely "what if"s or the fact that the half-vegetable person is able to drool and look around, keeping them alive can only be detrimental.
The next sort of category, is people who are genetically useless to society. They may have some higher order function, such as language, but by birth, they are unable to function as anything but dependants. The problem with this point, is that it's difficult to decide what we would have wanted done in their position. Not many would choose a life of pain and inability, but it's hard to say that incapability is enough to make life useless. From the perspective of economics and society, yes, but the problem is that we keep eight year olds alive, and when we observe that a particular individual might never pass that point due to genetic flaws, it is difficult to make the call. I am personally of the opinion that in this case, it would be legitimate to end the life at birth, in order to supply the parents with another chance at a normal parenthood, and to spare the person a life so inferior to the ones others are capable of having. If there are to be a limited number of people in the world, "re-rolling" the dice for those who are disadvantaged at the start seems to be a rational choice.
Now, since this reeks of "playing god" as it is, I would state that if there is a benevolent god, then (at the risk of intentional fallacy on god, if that even works) I would care to guess that it would be rational, and thus understand that if whatever meaning there is to human life would be acheived, we must strive to make sure that the limited space in the world is used for those who are able to fulfill whatever this meaning may be. From a Nietzchean standpoint, we of course see that those incapable of human expression do not have purpose, and from a materialist perspective it's largely similar to any argument showing that the greatest amount of happieness for the population can only be found when the population is made entirely of contributing members. That being said, concerning the idea of supposed inability to comprehend the divine, or to judge other people, I simply say that by letting someone live, we make as much judgment as we would by killing them. I know that the obvious counter argument is that we disrupt natural order by eugenics, but considering that we may choose when or with whom we reproduce, making an unlucky choice need not cost humanity as a whole. As a point, arguments such as "you could have killed the next ----" are inherently flawed because of the fact that the replacement to such an individual (which would inevitably be desired) would have the same probability of excelling.
As a note, once we hit the "wellfare class" I believe that before Euthanizing them all, it would be a better idea to impose harsher laws and get rid of the free ride for those who don't deserve it, instead of going so far as to kill people.
None.