Chosen answer:McCoy has no real idea how much blood he's going to need to bring Kirk back - given the catastrophic radiation damage to his body, there's every possibility that he might need multiple transfusions over a period of time, which would be much easier if Khan was still alive. Plus there's also the issue that killing Khan could well involve spilling some of the blood that McCoy so desperately needs. Bringing Khan in alive is the best way to maximise their chances.
Chosen answer: McCoy has no real idea how much blood he's going to need to bring Kirk back - given the catastrophic radiation damage to his body, there's every possibility that he might need multiple transfusions over a period of time, which would be much easier if Khan was still alive. Plus there's also the issue that killing Khan could well involve spilling some of the blood that McCoy so desperately needs. Bringing Khan in alive is the best way to maximise their chances.
Tailkinker ★