Dehumanization or the expulsion from humanity makes rights violations a lot easier – easier to justify, also to one’s own conscience – because there can be no human rights violations if the victim is not really human or less then human. A non-human does not have human rights.
If a person has rights because of his or her humanity, then it is important to recognize the right to belong to humanity. In the words of Hannah Arendt:
The right to have rights or the right of every individual to belong to humanity should be guaranteed by humanity itself. (source)
Many cultures, ideologies and religions have, at some moment in time, excluded certain classes of humans from humanity. Some still do. Some of the major religions have considered or still consider non-believers to be an inferior species, outside of humanity, not real humans and therefore not entitled to the same rights. All racist ideologies, by definition, exclude large parts of humanity.
Examples of dehumanization are here. I’ll just pass along this one, in order to show that dehumanization can start very innocently:
Read more about dehumanization here.