@Vincenzo -- a good question and one that is hard to answer because tort law would put anyone injured by that trap into the same category. It is pretty clear in the United States that the courts frown on devices designed to protect property by injuring someone. The view is easy enough to understand -- people are more important than property in the eyes of the law, so homeowners who set up devices to injure people will likely have to pay for their injuries whether or not the injured party was out to commit a criminal act.
Since that is the case, would a premises liability policy kick in if anyone was injured by that homemade trap? It seems the answer might be "no," but are there any exceptions?