WalMart drives out small local businesses, and replaces the lost jobs with very poor jobs in their own stores. WalMart is notorious for keeping almost all of its employees at less than 30 hours a week, so they cannot get job benefits. Even those who are hired full time cannot access health insurance (which they have to pay for) until they have been employed for two full years.
WalMart, because of its huge buying power, has been responsible for lowering living standards for a large number of laborers in China; WalMart's insistence on low prices means that many workers in China are paid for less than minimum wage, and are subjected to grossly substandard working conditions.
If you actually check carefully, WalMart's prices are not uniformly cheaper than competitors' - I have done comparison shopping between my local WalMart and a grocery store that is part of a regional chain. WalMart has cheaper "loss leaders", but for a great number of products, the regional chain is the same price or cheaper. A company called Zenith Consulting published a study that indicates that what WalMart is really good at is making consumers think that their prices are lower.
WalMart is a monopoly, which has been long recognized as a bad thing for the economy and the country as a whole. However the company has become so politically powerful that no one will do anything about it.