Sanger wasn't a racist. This is a lie that has been trumpeted by right wingers who have quoted Sanger out of context. The line that is repeated as evidence that Sanger was racist was from a letter to African American ministers seeking to expand birth control access in the African American community:
"We don’t want word to go out that we want to exterminate the Negro population and the minister is the man who can straighten out that idea if it ever occurs."
People quote the first have of the sentence as evidence that the purpose of birth control for Sanger was to reduce the African American population. But in context, she is clearly saying that she wants to work with African American ministers to dispel fears in the community that providing women with access to birth control was really just a secret plot to destroy the African American community.
Sanger opposed racial segregation and made providing family planning to the African American community a priority because she saw how white communities had good access to family planning and benefited from being able to control how many children were in their family so they were not overburdened economically. Martin Luther King praised Sanger posthumously for her work in the African American community.
Sanger's only sin was buying into a very mainstream version of eugenics. She did not see eugenics as a way of producing a master race. She did believe that family planning would help produce a better population by eliminating children who were brought into families that were irresponsible and unable to properly care for their children. She actually equated these irresponsible people with those who followed the strict anti-birth control teachings of the church than any particular race.