I am not God. I do not have the capacity to see through time and I did not lay out a design for the universe.
Sickle Cell Anemia is fatal, but it also has a benefit in that sickle cell carriers are immune to malaria. My friend's son has Aspergers and while he can't speak well, the kid is an electronics guru. His brain functions just fine, it happens to function differently than mine. He's sentient (which is my criteria for the life, liberty, property thing) and therefore killing him would be immoral.
If being a retard (in whatever form, for the sake of this thread) is a negative trait, it will extinct itself. I support no action against the retarded, and not artificial actions to support them.