This is a very interesting point. Alas, the left doesn't think rationally enough to figure this out. As I've said before, Darwinism appeals to liberals for the same reason running away from home appeals to children: if you don't think it through, it seems to provide you with a way to live life on your own terms.
It is enough for them that Darwin gives them a way to be their own gods. They don't realize that when they killed the root of the tree of morality and meaning, they killed the whole tree too. When you kill the roots of a great oak, the branches will still support you for some time.
The fact that without God there is no coherent basis for objecting to human suffering eludes them. It is very ironic to me that Darwin was vehemently against slavery; apparently even he didn't figure out all the ramifications of his own ideas.
Darwinism speaks only towards the development and diversity of life, not those of morals, ethics, or political policies. "You can't get an 'ought' from an 'is'." However, if you are actually interested in an answer to your objections from a Darwinian perspective, I highly recommend "The Moral Animal" by Robert Wright. Unless, of course, you enjoy making baseless attacks, in which case I would avoid any sort of scientific literature.