How sound are the hadiths reported concerning the killing of black dogs, and is the command to kill them consistent with the mercy of the Prophet, peace and blessings be upon him, and is doubting these hadiths considered a sin?

Scholars have unanimously agreed on killing rabid and aggressive dogs, but they differed on killing those that pose no harm.

The majority of jurists hold that the command to kill dogs has been abrogated, and that killing a dog that offers no benefit and causes no harm is disliked (makruh tanzihi), with some even deeming it forbidden.

The Hanafis do not permit killing non-harmful dogs. The Malikis believe that the prohibition against keeping dogs is general, except for hunting dogs, farm dogs, and livestock guarding dogs. The Shafi'is state that the command to kill them is abrogated, except for the pure black dog and the aggressive one. The Hanbalis consider it forbidden to kill trained dogs and any dog whose keeping is permissible.

That which is permissible to kill is the aggressive dog, and anything that harms people and damages their lives and property.