It's a very complex legal matter. I'm trying to desrcibe the situation shortly. (I'm not sure about the translation of all the legal terms.) It is legal to defend yourself according to the danger of the situation. Let's say, two men want to steal my money on the street at night. One is trying to hold me the one punches me. And let's say, I know karate. I have the right to defend myself so far, that they are not able to continue their attack. This may include breaking the attackers arm. BUT it would be illegal to continue hitting them if they're already defenseless.

The means of defense have to be appropriate. Let's say, someone is sitting on my cherry tree stealing my cherries, I'd go to jail if I shoot him.