Why not slaughter the SAME ONES? Might as well toss some organs into them to double their value.

I doubt you’d want an organ from one of those pigs, they’re treated and fed awfully

As i understand it, you would be facing the same problem as with cannibalism, i.e. increase transmission of prion diseases, from the human tissue in the pig.

These pigs are specifically genetically engineered and bred so as to be compatible. There are viruses, etc. that are endemic to the swine population that would kill people. It happened in that first human implant recently.

Sacrificing any animal would be morally justifiable if it saved a human life, as is the case with kidney transplants. It does not matter that we already breed and kill millions of pigs for meat consumption. The fact that some person with end stage renal failure could be saved, return to full health and live normally justifies the means. The species (other than human) does not matter - pigs, chimps, whatever allows is to accomplish this scientific breakthrough the fastest.

Where ethics come into play is how the animal is kept during its life. And whether we prevent suffering when its time to remove the kidneys and kill the donor in the process. I would speculate that in the interest of good donor organ function, we would make sure the animal does not experience any stress.