What is 10th juror's bias?

In the play, Juror Ten is violently prejudiced against anyone who comes from a slum. "You can't believe a word they say," he says early in Act 1. Note, that he does not say "he," meaning the defendant, but "they," the group as a whole, which shows that he cannot make a fair judgment about individual guilt. Juror Nine, the old man with much experience of life, sees this immediately and rebukes Juror Ten ("Since when is dishonesty a group characteristic?"). But Juror Ten's bigotry continues to smolder before finally erupting in a long speech near the end that leads the other jurors to reject him. The message is clear that such irrational prejudice is incompatible with justice.