I think it’s the problem of being a relatively more pointless waste of life. Eating a whole animal at least provides substantial nutrition. Also the fact the animal is left to suffer for hours or even days, motionless and alone at the bottom of the sea. There’s something excessively inhuman about simply abandoning life like that.

We can also recognize that prisons today are tortuous and should be improved (much like the farming/slaughter industry), and the death penalty is hard for many to justify as well, but paralyzing a human and throwing them in a hole to rot while still alive—even if they were a major criminal—would probably outrage nearly everyone.

Substantial nutrition that we don't need though. You could get even more nutrition if you didn't feed the animal in the first place and just ate some crops made on the same land the animal would have eaten.

It's like those golden tomahawk steaks. There's no point in covering a steak with gold, yet we spend effort digging a hole in the ground to do it.

Unfortunately, humans explicitly evolved to consume animal products. There are B vitamins that humans can only naturally acquire through eating animals that we die without.

It’s possible now to generate those B vitamins, but saying humans don’t need to consume animals is like saying we don’t need to walk to be healthy. Technically true, but most of us have two legs that we evolved to live a human lifestyle and getting around that requires a bunch of atypical lifestyle changes that are massively difficult and unnatural for most people.

People don't eat animals to get B vitamins.

If they did they wouldn't need to eat the amount they do.

The animal might not care but we should. There's a sustainability factor that should be included in decision making around harvesting product in general. Marijuana is a decent example. Once turned to flower, the whole pant gets killed for harvesting. The product (flower) is worth a lot more than the remaining plant.

With shark finning, the product (fin) is worth less than the shark even by the most conservative estimate.

If killed, would you choose a swift painless death or someone chopping off your limbs and leaving you too die slowly over next few days.