What is so wrong with Eugenics?

So after reading the thread about NASA's budget being cut, and having heard about it for a while now, I started thinking about the future of this planet and what it would mean to actually get in contact with aliens and survive as a race and so on.

Basically, Evolution has stopped working. Stupid people will continue to procreate and will at a higher level than smarter people for the simple reason that they are stupid and like to have unprotected sex without considering the consequences.

What this leads into is the question of why Eugenics is so bad. If we want our race to survive and come together as a whole planet to solve our problems, we're going to have to eliminate the unintelligent that are holding back the race. Now I'm not saying that we should necessarily sterilize individuals, but what is so wrong with altering DNA, genetics or enhancing genes to make future generations smarter, stronger and more able to unite the planet?

**EDIT: I don't think half of you are reading my full post. I don't mean killing off or sterilizing anyone. What I mean is altering the genetic DNA of a family line so that future generations stemming from that line do not inherit the negative genes passed along that line and can be injected with genes that have been deemed as heavily desirable.

SECOND EDIT: So some of you have pointed out, perhaps Eugenics is not the right term, and it would be more accurately described as genetic engineering or modification.**