You have sinned for hiting an animal–Shaikh Abdullaah Bin Muhammad Bin Humaid (rahimahullaah)
A man hit an animal and broke its hand, and thereafter the animal died; what is the ruling on this?
We do not know which animal it is, and whether it is one of importance or of a valuable price. So what is intended here is that you are not held responsible except that it is impermissible for you to punish an animal. You have sinned in the sight of Allaah and it is incumbent upon you to repent. As for kaffaara (i.e. a prescribed act of expiation for this sin of yours), then there is none incumbent upon you.’’
[Source: Al-Fataawaa Wad-Duroos Fee Masjid Al-Haraam, page: 925]