As noted, not slavery.
In addition, if Wal-Mart treats their employees so terrible, why do they work there? With slavery, they would be forced to work there. They aren't, they can go seek different employment anytime they want. They are likely there because they don't have any marketable skills to get a job anywhere that pays more.
In highschool, I worked for a restaurant. A couple people quit that job so they could go work at Walmart instead. It was a $0.50 per hour pay difference($8.00 VS $7.50/hr), which they thought was awesome at the time. The restaurant gig was a fun job. However, working with people twice my age was a huge reminder that I had to finish HS and get a college degree, because I didn't want to still be there 20 years later.
Minimum wage and barely above minimum wage jobs pay very little. I see two solutions. 1. The employees acquire marketable skills to get better jobs. For many, this means finishing high school. 2. The rest of us pay significantly more for purchases at Wal-Mart, grocery stores, restaurants, etc. Or, we have UBI and we all pay higher taxes. People at different ends of the political spectrum might like one answer over the other, and I'm not trying to start a debate between the two... I just don't see any other solutions.
Some math and perspective: WalMart CEO = 22.2 million. Average employee= $19,177. WalMart has 2.3 million employees. If the CEO was paid $0 each employee would get a $9.65 per YEAR raise. Yes, he is paid a lot, but also responsible for millions of jobs and hundreds of billions of dollars. If Doug(CEO) makes smart decisions and WalMart grows, then those 2.3 million employees likely have more job security. If he makes bad decisions and Walmart goes under, then those 2.3 million employees likely would be happy to pay $10/yr to still have their jobs. I personally, would be happy to pay $10/year to have an awesome CEO over a mediocre CEO.