Confusing, I grant you, but...
1. (Despite some misconceptions on this point, even among certain natives who should know better) there is no requirement in English for concord of number between a subject and its complement.
2. On account of a linguistic phenomenon technically termed 'synesis', usage, particularly in BrE, allows for a formally singular noun that refers to a group of persons (e.g. family, government, etc.) to govern a plural verb.
A people who are slaves
is perfectly acceptable!