Let's take two people, Alice and Bob. Alice offends Bob in some way and Bob is determined to take revenge. He does this by starting new businesses to compete with Alice, e.g. if Alice owns a restaurant, he starts a new restaurant nearby, sells similar food, and sells it more cheaply (because he's willing to take a loss to get revenge, and he has the resources to sustain the loss). If Alice moves locations, he moves too. If Alice switches businesses, he switches too.
Is it legal for Bob to do this kind of thing? If it violates a law, which one would it be?
This kind of behavior sounds obnoxious enough that I'm guessing it is illegal, but I can't find a law that prohibits it. It doesn't seem like harassment, since Bob isn't humiliating or embarrassing Alice. It doesn't seem like unfair competition, because presumably once Bob achieves his aim of driving Alice out of business, he won't raise prices to unreasonable levels, in fact he'd probably shut up shop too. I'm tagging the question with "harassment" and "competition" anyway, since I can't think of anything else.
If the jurisdiction matters, assume the EU. If the country matters, assume Germany.