Evolutionary theory goes all the way back to some of the first pre-Socratic Greek philosophers (e.g. Anaximander, Empedocles, etc...); however, Darwinian evolutionary theory was critical in influencing the development of Nazism and Marxism.
Socio-political theorists like Karl Marx, Emile Durkheim, Max Weber and a number of forebears, associates and successors employed Darwinian evolutionary theory within the context of atheistic reductive materialism to build societal models.
Their models were instrumental in the development and execution of 20th century Marxist Communism and Nazism which resulted in sweeping democide, genocide, persecution, historical destruction, etc...
There were radical political extremists, of course, before Darwin published his seminal work 'On the Origin of Species' in 1859; however, because Christian principles were deeply rooted in Western Civilization they were unable to persuade the masses to accept their ideologies.
Christianized Western Civilization is often accused of dampening scientific progress by non-Christians. Political conflicts between scientific theories and the Christian faith have arisen through the centuries but the level of conflict has often been exaggerated and Christianity's positive influence on scientific progress is seldom acknowledged by those making these claims.
The intellectual climate that gave rise to modern science (roughly three centuries ago) was decisively shaped by Christianity. Not only were most of the founding fathers of science themselves devout Christians (including Copernicus, Kepler, Galileo, Newton, Boyle, and Pascal), but the Christian worldview provided a basis for modern science both to emerge and to flourish.
Christianity didn't have the problems many other worldviews had such as a cyclical approach to time, an astrological approach to the heavens, metaphysical views that either deified nature (animism) or denied it (idealism), etc...
It is for these and many other reasons that Western nations were able to block the most radical movements for centuries from creating the destruction they finally did create.
Darwin, however, opened the door to uncoupling Biblical morality from society ushering in a ‘scientific’ rationale for socio-political models built on state atheism which inspired radical political extremists to create new socio-political models based on state atheism while affecting hybred one's like Nazism which incorporated pagan and occultic elements as well and others besides.
The result was horrorific bloodshed and suffering and those atheistic socio-political models that despite the fall of the Soviet Union, Nazi Germany, and other regimes built on these socio-political models still negatively affect large numbers of people today in places like China, North Korea, etc... where they are maintained to one extent or another.
Social Darwinism is built on atheistic reductive materialism as the only real metaphysical worldview and it is for this reason that it is diametrically opposed to Christianity.
Christian sociology competes against social Darwinisim. To understand Christian socialism, simple primers are available like 'Sociology: A Christian Approach for Changing the World' and 'Total Truth Liberating Christianity from its cultural captivity.' Of course, a proper exegesis of the New Testament is instrumental.