A better genotype does add value to a person's existence - For example, all other things being equal, a healthy person both individually feels better and is better for society than the 'same' person with a genetic defect that causes a horrible death in his/her childhood. If we can fix that, then it's a noble cause. If a genotype doesn't add value to existence, then it can't be called a "better genotype", then it's simply a different genotype.
And, by the way, "If X, then eugenics is not that bad' is in no way an acceptable argument against or for X; it's worse than irrelevant.
