Eugenics is defined as the betterment of the human gene pool through selective breeding.
The fact that genes play a significant role in behavior, intelligence, and illnesses such as cancer and cystic fibrosis is pretty well established. What would be so wrong with sterilizing people with mental retardation or serious inheritable diseases? Is it not cruel to let a child be born with a 90% chance of getting, say, Huntington's disease?
If I were in charge, I would first sterilize all violent criminals. Yes, there is a small percentage of people in federal prison who are innocent, but the vast majority are not. Raising a child is one of the most demanding things a person can do, and a violent offender will more than likely not be able to stimulate the child's curiosity in a positive way or teach them moral lessons or anything else of the sort. This creates a cycle of crime and poverty that is difficult or impossible to break out of.
The mentally retarded or borderline retarded should be sterilized as well. The boundary should be at an IQ of 80; if yours is below 80, you will be sterilized, and if it's higher, then you won't be sterilized. Intelligence has a strong genetic component and people with low intelligence will probably be unable to raise moral and intelligent children. I realize IQ is faulty and arbitrary, and saying that a person with an IQ of 122 is more "intelligent" than someone with an IQ of 120 is ridiculous. What isn't ridiculous is saying that a person with an IQ of 100 is clearly more intelligent than someone with an IQ of 80.
If you want to have kids, fine; there are plenty of them up for adoption all over the world. People have to realize that there are things more important than their own personal desires; the maintenance of the human gene pool is one of them.