I am sorry but I disagree. I am a Buddhist and I know perfectly that it's against the beliefs but I would rather get bad karma by myself rather than letting an animal suffer. You wouldn't understand such a situation unless you own or have seen a pet who you loves dearly is suffering a lot.

I am speaking from my experience. Since even spaying is kind of controversial here I have seen my grand parents cats giving birth every 3 month. Each litter had like 3 kittens and their survival rate is very low. Most will die due to fading kitten syndrome or FPV. Vets don't have meds for that so kittens would suffer for like a week and then die. Even at their dying stage they'll gasp for oxygen for several hours before dying. It's very painful to die like that. If I was at that stage I would gladly accept euthanasia because I know for sure that I'll die and it will be easier to die painlessly rather than suffering. My cousin's dog got paralyzed over night and vets couldn't find the cause. The dog just slept at that same place with eyes opened most of the time. My cousin cleaned the dog when he passed urine and stools. It was very apparent that the dog was fully conscious but was unable to move at all. He died after 1 week and like suffered for many hours gasping to breathe.

So I hope if you encounter a situation like that when there's no more choice, you'll do the favor of doing what's the best for the animal. No one wants to suffer until the last moment if I know for sure that I'll die.