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 voice for speaking out against the atrocities they are forced to commit every hour, but we can change that.
The statistic on the number of land animals killed every hour is derived from the 70 billion animals that we kill each year for food. Indeed, we kill more animals in one hour than the Nazis killed Jewish people during the entire holocaust.
The oppression of the slaughterhouse workers is detailed in this New Yorker article.