There are so many reasons to hate walmart... so many.
First off they create ghost towns all across small-town America. Walmart will move into a small town, which drives every other independent business out of town. Then when the walmart store doesn't turn enough profit, they close the store. Now we have a town with no stores. How would you like to drive 50 miles to get a gallon of milk?
Even if the store doesn't close down, Walmart still wrecks havoc on local economies. The former business owners that were put out of business by walmart would of spent the majority of their income in their local communities. Their high paying jobs are replaced with minimum wage jobs. The difference in money is then shipped to Walmart HQ to pay their bills or whatever, and taken out of the local community.
Due to the sheer size of walmart, they have crazy buying power. Their buying power allows them to bully companies into selling products to Walmart too cheap. Walmart builds a relationships with a company, paying a fair price at first. When that company relies on Walmart's business, Walmart buyers will then beat them the hell up on the price. It becomes a catch-22 for companies that sell to Walmart: quit selling to Walmart and go out of business, or keep selling to walmart and go out of business because you aren't making enough profit per unit. Walmart has been one of the leading factors in several companies going belly up.
Then there's always the accusation of being sexist, the poor working conditions of warehouse workers, the near lack of health insurance, etc.
A better question is 'how can you not hate Walmart?'