Would you eat burgers if you did have to kill cows?
You don't eat people, but they still need to be killed?
I am not that crazy about burgers, but I would definiatelly kill chicken! Actually, if I get a machine gun, I might kill cows too....
Prisoners, yes, they still need to be killed for their organs, and your argument about their health is silly. If we can check normal donors for every disease, why can't we check prisoners?