One can observe bad management anywhere. I read a blurb about a gay Starbucks employee being harassed by his boss and coworkers just this week. I have had bad managers myself. Bad people exist, (and sometimes good people do bad things)
It is not fair to fault the company as whole, for a bad employee, (managers are still employees after all). Rather than boycott walmart, and spend more money elsewhere for the same cheep goods, why not call/write the corporate office about your concerns. Your feedback, properly channeled, could help the employee, the local store, and the company as a whole.
I do not believe it is walmart company policy to treat employees like crap. In fact, to avoid "going walmart" replacing "going postal" as the colloquialism for a shooter on the loose, it is mostly likely policy to do the opposite.