Darwin was always afraid that his theory would be used incorrectly. He was speaking of Darwinism as a microcasm of evolution and not this thing called "social darwinism". The term "survival of the fittest" is also inaccurate. It is more accurately survival of the most adaptive. An organism can be the strongest, largest, fastest thing out there but all of those functions can work against survival when being smaller is better, and freezing in place is preferrable to moving fast and giving away your position.
Historically reality; George Bernard Shaw, the English writer, was a member of the Fabian Society. He went public in saying that every couple of years a person should be forced to justify their existence. If they fail then they will be painlessly put down like an animal.
http://www.youtube.com/watch?v=OBZsTf6oLfY He was also a supporter of the concept of the Holocaust if not the execution.
Now we have some ethicists proposing after birth abortions. They theorize that small babies (not sick babies) have no personality yet and are just a blob of flesh.
http://www.slate.com/articles/health...anticide_.html
Be careful of the extreme left. They have a love affair with death.