The problem with eugenics is that it's completely scientifically invalid. And what starts with the (false) notion that you can measure someone's intelligence by measuring the circumference of their head ends with abominations like the Buck vs. Bell decision upholding the legality of forced sterilization as a cure for "hereditary feeblemindedness." In this decision, Associate Justice Oliver Wendell Holmes actually stated "Three generations of imbeciles are enough." (http://www.eugenicsarchive.org/eugenics/topics_fs.pl?theme=3)
That quote should, in and of itself, answer the question of what's wrong with eugenics.