Sometimes, this excuse has been "Christianity". But Christianity was not the cause. If it was done by Christians, citing Christian scripture in justification, Christianity was in the same sense the cause, as a fallacious reading of evolution that led people to social Darwinism.
Please see my reply below and find that it is less than a fallacious reading of Darwin that allows for such. As far as "Christians" doing these things, I just don't believe that these people were Christians- at least not in the biblical sense. We can disagree on that point.