It is not only a despicable act, but also an act of provocation which might force the animal to retaliate. Some people resort to harming animals because they steal or are mischievous. But, we don't deal with naughty children by pouring boiling water over their backs. Or, kicking them till they bleed. Or, gouging out their eyes.
Humans seem to lose their minds when it comes to educating animals.
It is almost like the hideous animal traits in them emerge. Harming defenceless creatures is just as bad as harming an innocent child. An animal doesn't even have the ability to cry and express its pain. It deserves an equal amount of compassion. From another perspective, we have pet owners who like keeping and displaying animals like trophies.