Character mistake: During the scene with Kirk and Khan "space-flying" over to the Vengence, Bones loudly tells Kirk that he's "off course", with Karl Urban mistakenly falling into his New Zealand accent when saying "course."
Continuity mistake: Just before Spock and Kirk go to seek out Harrison, Kirk is examined by Bones. During the examination Bones is wearing a pinky ring, it's then gone, and then back again.
Other mistake: After the Enterprise is disabled by the Vengeance and falls out of warp, Kirk and Spock have a discussion in one of the corridors about letting Khan help incapacitate the attacking ship. Watch behind them. The ship is running under Red, but the crew members are walking past them calmly, some even watching their conversation as they pass! Not really Red Alert protocol... In fact, through all of this, only one crewman runs past in the entire sequence.
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 ★