Then I have an "easy" patch: let the entity that does the spotting and killing be incorruptible and infallible. Like, an AI, an army of robots, or something. With that, I don't see any obvious flaw beyond the fact that, with this level of technology, there are very probably better alternatives than transplantation. But the idea of creating a machine for the explicit purpose of killing people might be even more creepy than the police state we're vaguely familiar with.

Compare with the comment I saw somewhere with this dilemma:

• (a) Let the aliens that happen to visit us cure cancer, except for 1 random patient out of 100, that they will let die, then eat.
• (b) Just let the aliens go, never to be heard of again.