What is the ruling on harming and killing pet animals such as cats and dogs, how can one atone for that, and is repentance accepted for it?

It is forbidden to intentionally kill cats and dogs—other than the all-black, aggressive dog—without a legitimate Shar'i purpose. The one who does so is sinful and must repent and seek forgiveness, for the Prophet, peace and blessings be upon him, said: "There is no human being who kills a sparrow or anything larger without its right, except that Allah, the Mighty and Majestic, will question him about it." It was asked: "O Messenger of Allah, what is its right?" He said: "To slaughter it and eat it, and not to cut off its head and throw it away."