Here is the Swiss law about self-defense (my translation). I find it very clear, and was impressed by the fact that is takes "going too far" into consideration, and is ready to excuse it.
I'd be very interested to know your thoughts about this law, and how it compares with the law in your country.
Legitimate defense / Art.15
Anyone who is, against the law, attacked or threatened by an imminent attack, has the right to repel the attack by means proportionate to the circumstances ; the same right belongs to third parties.
Excusable defense / Art. 16
If the author, while repelling an attack, has exceeded the limits of legitimate defense in the sense of art.15, the judge alleviates the sentence.
If this excess comes from an excusable state of excitation or of shock caused by the attack, the author is not acting in a culpable way.
State of licit necessity / Art.17
Anyone who commits a punishable act in order to preserve from a danger both imminent and impossible to avoid a property belonging to him or to a third party is acting in a licit way if he thus saves preponderant interests.
State of excusable necessity / Art. 18
If the author commits a punishable act in order to preserve himself or someone else from a danger both imminent and impossible to avoid that threatens the life, the corporeal integrity, the freedom, the honor, the property or other essential goods, the judge alleviates the sentence if the sacrifice of the threatened good could reasonably be required from him.
The author does not act in a culpable way if the sacrifice of the threatened good could not be reasonably required from him.