Is it permissible to cut off a part of the tail of a guard dog, and what is the ruling on doing so?

Islam is a religion of mercy and compassion, forbidding the torture and harming of animals. Therefore, it is not permissible to cut off a part of the aforementioned dog's tail, as it involves torturing and harming it. What matters in this regard is the ruling of the Sharia, not customs that contradict it.