The only person you're "letting die" is yourself, and not even that's 100% certain since bloodless surgery is still an available option, if you're a Jehovah's Witness.
Methinks you're being a bit economical with the truth - sure it's an option, but you forgot to mention that it doesn't work nearly as well. Do you think they'd waste all that time and money desperately campaigning for blood donors if they didn't actually need it?
You might be saving a life if it works, yes, but the exchange is simply not worth it in the long run if you believe in God.
Fictional or not, Christ's defining characteristic is supposed to be compassionate self sacrifice, and you dare to assert, in his name, that it is not worth forgoing one's heavenly reward to save the life of another? The whole point of the promise of heaven, childishly simplistic though it is, is to try and induce sociopathic wretches like yourself to act selflessly by offering greater, delayed compensation for their immediate perceived loss (thus turning their own greed against itself), not to reinforce their indifference! Clearly, it's backfired in a big fucking way here.
The concept is by no means unique to Christianity either; it's a classic component of any number of myths and fables across the globe: the protagonist dilligently obeys all the rules and meets all the requirements he knows he must in order to get whatever fabulous reward the particular story is about, then is suddenly faced with a morally unacceptable challenge, the decent, humane response to which unavoidably breaks said rules and forces him to forfeit everything. The hero then usually agonises for a while, desperate to get the fabulous treasure and not to have wasted all the effort that got him this far, but then falters at the last minute, and prepares to leave in shame - whereupon the deus ex machina announces that this was actually the correct answer all along and that he's won, by demonstrating his compassion, restraint and mastery of his desires. The basic moral is, of course, something along the lines that only those who would be willing to go without reward, comfort, riches, heaven or whatever, who exercise self restraint in their pursuit, actually deserve to have them; that only those who would accept failure deserve success. It's worth noting that the notorious Milgram experiment is a more modern expression of a very similar idea.
By the way, the story of Abraham getting ready to kill his own son at the arbitrary whim of his God is the only example I can think of, offhand, of a fable of this kind that gives the wrong fucking answer; where the protagonist is actually rewarded for failing to exercise any personal restraint or moral strength and acting in total self interest! A lot of people reckon the bible's a mish-mash of borrowed concepts and stories from earlier cultures; but I reckon the authors not only stole this particular one, but got it backwards when they rewrote it, either through sheer incompetence or as a deliberate means to reject the morality of the earlier source.
And wait a fucking minute, I just realised another of your central tenets is that you're supposed to be strengthened by the damn blood of Christ yourself! The single most important ritual of most christian denominations is a symbolic blood transfusion, so how the hell can you oppose real ones???
