Sometimes, yes. Selling something for below cost with the intent to drive your competitors out of business is called "dumping" and has been illegal since the 1800s.
Because selling a product below cost is fundamentally unsustainable, there is no logical reason to sell a product for less than cost besides doing so temporarily with the hopes of being able to later recoup the loss with higher, above cost prices. This is anticompetitive because an inferior product can win out if it is backed by bigger pockets that can afford to stay unprofitable longer than the company making the superior product.
This is basic economics, not really something that needs to be thought out and debated from scratch by HN over and over everytime it comes up, so it really would be helpful if everyone who is thinking about commenting on economic issues like this tries to at some point spend a couple hours reading an AP Microeconomics text. If a high school kid or college kid can do it in a semester, and intelligent adult can cover the high points in a weekend.