Answered by Shaykh Abdurrahman ibn Yusuf Mangera
A family has a pet cat which is dying of cancer, the cat is in terrible agony. Some members want the cat to be put down by a vet to end its suffering while others argue that this is against Islam since you are taking a life and if you can do mercy killing for pets what prevents mercy killing’s on a human being suffering from cancer. Also can a family spay their pets to prevent unwanted animals. What is the Islamic viewpoint.
In the name of Allah the Inspirer of truth.
1. It would not be permissible to have the cat killed even though it is in agony. It is a living soul and its death should not be effected by its owner. It should be allowed to die its natural death. However, if the animal is close to dying and there is no chance of recovery it could be slaughtered to relieve the pain. [Radd al-Muhtar 5:305]
2. It is permissible to castrate animals if some benefit is derived thereof, for example the animal becomes more healthy or its meat tastes better, etc. or it can save from some harm. In the case in question if there is nowhere to send the kittens and they are becoming a nuisance around the hose or locality then it would be permissible to spay it. [Fatawa Hindiyya 5:357]
And Allah knows best.
Abdurrahman ibn Yusuf