Popa, I don’t assume you mean in a case of severe pain as well. See R’ma E.H. 5;19 that although the Halacha is that it’s always muttar, in a case of extreme pain, the Minhag is to keep the Chumrah to abstain.
It’s the improper balancing, and maybe the consideration of death, which is a shtus, but even for human benefit, we don’t disregard the animal’s pain entirely.
I hope this answers some other posters’ replies to me as well.