Unfortunatly hoarders really DON'T understand what they are doing. They feel that if the animal has lived that way forever, that the animal knows nothing different; Therefore, it has nothing to be sad about. That the way they live, whether it's sick, dying, in pain, whatever, is better off than being dead -- While a normal person would be able to end the animal's suffering, an animal hoarder feels that death is the worst outcome.
I know because my grandmother is a hoarder. She has had literally hundreds of cats, and dozens of dogs at a time. Chickens, geese, guinea pigs, rabbits, etc.
When she moved away (out into the bush) the forest fires had driven alot of animals into her area, so the coyotes etc did away with most of the cats, which was actually lucky for us because we were able to spay all of the females but two, and neuter all of the males but a three or four. That was years ago so she has probably built up yet another army of them by now, not sure.
She heard me bashing puppy mills once and flipped out, told me that the dogs didn't know that life could be better so that is just fine for them -- how else can you get a cheap dog (even though pet stores charge outrageous prices)?? Said the same thing about over 50 husky/mal/shep mixes that were chained up in her area, suffering from giardia, most of them being PTS -- she completely flipped out. The dogs had always been there and were happy, so she didn't think it was right to take them away from there. They didn't know better.

The last time I went to her house, she had cat feces on her counter, in her bath tub, and on her bed. She knew we were coming over.
She smells so badly like a litter box when she comes here. Anything she gives us needs to be washed several times to get the smell out.

She always had too many animals -- but it didn't get really bad until my grandpa left her. Then she went out of control. My mom and aunts are very resentful. My mom remembers her friend waiting on the steps to her house for over an hour so my mom could come play -- but she wasn't allowed to go play until she was finished cleaning the cages. She had to scrub them all. Over an hour later, her friend had to go home for dinner, and my mom wasn't done yet. The kids were responsible for the animals.
One time she and my aunts walked a long ways to take a litter of kittens to the vet. The vet said the kittens were suffering and would likely die anyways -- they needed to be put down. So they left the kittens there. When they got home, my grandma was so furious, she flipped out and made them go back and pick the sick kittens back up again.
Years later, my dad recalls her having too many vehicles -- a long driveway, with several vehicles, and a dog tied to each one. He specifically remembers a dalmatian tied up on a short chain to a van, seemingly going out of it's mind.
I remember when one of her dogs got loose and cut itself -- a huge gash in it's neck, you could see all her insides. She let it stay on the chain because she couldn't take it to the vet until the next day. I could just remember the collar touching the gash and it grossed me out so bad. She didn't really seem to think it was that bad.
That is the problem with hoarders, it's never THAT BAD. To them, it's just the only way the animals have lived, and life is better than death... So to them it's perfectly rational.They don't realise that they are thinking of their own feelings, and how difficult it is for them to lose those animals; They don't consider that maybe it's in the animal's best interest to BE lost...They just refuse to think that way.