What is the ruling on killing a black dog—acting upon the hadith—if it is a police dog used to help uncover criminals and explosives?

There is no disagreement concerning the permissibility of killing harmful dogs. As for non-harmful dogs, many Maliki scholars are of the opinion that they should not be killed unless they are causing harm, and that the command to kill dogs has been abrogated. They base this on the hadith: "Do not take anything that has a soul as a target," and the hadith about the dog that was given water, and the Prophet's (peace be upon him) saying: "For every wet liver, there is a reward." Meanwhile, Shafi'i scholars view it as disliked to kill an animal that shows neither benefit nor harm, and Hanafi scholars hold the same view. The summary of the matter is to consider this difference of opinion and not to kill a black dog if it is trained and owned, especially if it is owned by the state and used for public interests.