In terms of primary and incidental, it depends on the type of prisoner. If it's a prisoner sentenced to death, then it would be incidental. They're going to be executed anyway. To a certain extent, it started off as incidental and then became primary, partly because the system had already been set up; it's just a matter of shifting.
The Falun Gong and the other prisoners of conscience, but particularly Falun Gong, were very heavily vilified with this incitement to hatred, which depersonalized them, particularly in the eyes of the people in the state system, the jailers, who tended to buy into this propaganda.
The Falun Gong are normally not sentenced to anything. They're certainly not sentenced to death. Some of them are sentenced for disrupting social order and get a three-year sentence, but a lot of them aren't even sentenced to that. Falun Gong, although it was repressed by a Communist Party decision, was never legally officially banned. You don't violate a law by practising Falun Gong. You just violate party policy, which of course is above the law.
When it came to the killing of Falun Gong for their organs, that wasn't incidental. That was primary. Otherwise, they would still be alive, and many of them are still alive in arbitrary detention in China as a vast forced organ donor bank.