Slaughterhouse workers are oppressed by the same system that kills 8 million land animals every hour, 24 hours a day, 365 days a year, for profit. These workers have no voices for speaking out against the atrocities they are forced to commit every hour, but we can change that with our choices at every meal.
The statistic on the number of land animals killed every hour is derived from the 74 billion animals that we kill each year for food. Indeed, we kill more land and sea animals in one minute than the number of civilians murdered by the Nazis in one year.
The oppression of the slaughterhouse workers is detailed in this New Yorker article.