Why would it be wrong to breed out genetic defects?

Ehhh, the probably with eugenics is that these results can be a little unpredictable, and not always have the desired effect. For example, consider sickle cell anemia. It's a terrible illness, right? Your life expectancy goes down, you're in pain all the time...it sucks. BUT - it confers a certain immunity to malaria, which can cause anything from diarrhea that makes you wish you were dead to severe brain damage to death.