Morrissey sometimes writes movingly on the death of others who have meant a great deal to him. However, if he casually wishes death upon strangers or acquaintances it devalues his own eulogies. It also makes it impossible for him to protest when others wish death upon him, or those close to him, because it contributes to a lowering of civility in general.
As others have noted, it simply doesn't make sense to care for some animals but not others. Wishing death upon bull-fighters doesn't actually save any bulls. Wishing the IRA had murdered Thatcher may be a 'rebellious' soundbite but ignores the suffering of innocent people who were caught in that hotel bombing. There are so many instances of Morrissey issuing tone-deaf statements about the death of other human beings that is raises the question of whether his concern for animals is merely because he is so bitter and angry at other humans for his dire emotional life that he has become a hateful misanthropist.
I believe he is a teenager trapped in a middle-aged man's body and has never developed the emotional impulse control that normal adults have as he went from a hermetic sealed life on the dole to a hermetic, sealed life as a minor celebrity, surrounded by flunkies who he'd sack if they challenged his emotional incontinence.
best
BB