What is the ruling on one who killed a donkey that was destroying his crops and fruits and causing him harm, and is expiation required for that?

The basic principle is the impermissibility of harming animals, except for what has been explicitly permitted by a text, such as the five noxious animals (al-fawasiq), or if the animal is harmful and its harm cannot be repelled except by killing it. In such cases, it is permissible to kill it to ward off its aggression, and there is no expiation for that outside the Haram (sacred precinct).