Oh man I could write a novel about Walmart for you. I worked there as one of my first jobs and watched as adults were locked in until cleanup was done while a manager guarded the door - it was that shitty of a place to work that they literally locked employees inside. Luckily as someone under 18 I was forced to leave at 10am. In fact, there were many lawsuits about workers not being paid for their overtime and I've heard stories along these lines (being forced to clock out then finish collecting carts for instance).

Entering Walmart, I was indoctrinated in anti-union rhetoric. Many of the people working there would benefit from unions - they aren't paid particularly well and often are left to rely on public health assistance. The thing is - America doesn't have a public healthcare system. Here, health insurance usually falls to your employer. So Walmart has a history of passing this cost on to the community and not bearing it themselves. Many Walmart employees are also paid low enough wages that they actually make up large numbers of social welfare program recipients.

Walmart has a history of coming in and undercutting stores that specialize in a product (your local pharmacy, pet store, etc). After moving in and decimating local shops, they tend to lower wages and bump up prices. Gone are your $5 prescriptions - you don't really have a choice anymore since they were able to strong arm the little guys out of existence.

Walmart has poor practices when it comes to sourcing material. This includes past use of child labor and pushing "rolling prices" which in the end burn suppliers for Walmarts inability to sell product.

Finally, Walmart has been tied to a negative impact on a community despite the low prices they do bring on some types of product:

https://en.wikipedia.org/wiki/The_Wal-Mart_Effect

Walmart likes to push the benefits to their existence but on the whole I think most people feel we'd be much better off without it. I try pretty hard to avoid the company and purchasing anything there and like many Americans feel strongly against the company. If you want to see why Americans don't like Walmart, compare it with Costco which is the polar opposite to Walmart in nearly every way.