In a strict utilitarian analysis, they are saving one person's life, but could probably save several peoples' lives and dramatically improve the quality of life of others.
One utilitarian perspective is that a world where you are at risk of your vital organs being reallocated at any time is such a bad world (deep anxiety for everyone all the time) that organ seizure would never be a reasonable policy choice.
On the other hand:
(1) Maybe this isn't true and we just think organ harvesting is an evil because of our innate status quo bias. Maybe like in Kazuo Ishiguro's 'Never Let Me Go' if there were a class of people who could be harvested at any time they would just ... Accept it and deal with it.
(3) what if there were one person who just LOVED forcible organ harvesting, loved it ten billion times more than every other human on the planet hated it? Morally a utility maximizer would choose to bend to that monster's preference.
You're trying to make this polarized, but it's not about good vs evil. It's an observation that it's likely something in an equilibrium today is likely to shift somewhat in the future. No one is saying "now we need to harvest organs" but "there is going to be a new problem emerging in the future, and that problem will require new solutions".
Even as someone quite interested in autonomous vehicles, and optimistic that they will become extremely feasible, creating dramatic changes in our world in the process, I hadn't until now thought of the organ shortage they might create.
Does it mean we start creating people to harvest their organs? Of course not; please don't be ridiculous. Perhaps it means that biotech companies have yet another gap in medicine to think about tackling, though.
It's certainly not reasonable. But neither is responding to a question about the side effects of fewer donations as a result of car crashes with a dismissal about how the people in the cars will be alive.
Is there an externality that should be priced in here? How?