Technically, Hitler was not very good at understanding social darwinism. He twisted it. It's not so much about society oppressing people as much as saying that one can either a) let the weak die, or b) "help" and "civilize" them. The "civilizing" aspect can be used to justify oppression, but Hitler's was too artificial for social darwinism. His was racism, with even less logic (not that social darwinism is that logical anyway)
That being said, Darwin himself considered the theory beyond absurd. The theory doesn't take into account that individuals who are not that self-sufficient can still be useful (think about a genius in a wheelchair. they need outside help, but they can help others just as much). Also, it doesn't acknowledge either compassion or a desire to feel superior. Studies have shown that people measure wealth and position relative to their peers. Unless someone is impoverished, they tend to think they are poor or rich based on the people they know, even though those people are not an accurate random sample of the population. Do keep in mind though, that capitalism does, in a way, endorse a milder form of the idea. Essentially, every person for himself or herself, without help from others. With other constructs such as unions and helping the impoverished, this is lessened.