I'm certainly no lawyer, but I have long heard about the legal principle of "You take your victim as you find him."
That means that whatever happens to someone as a result of an accident you cause or action you take, you are responsible for those injuries and/or losses even if another person would not have been affected to such a degree. For example, say you carelessly rear-end a car. An elderly person might have serious injuries and even die, whereas a younger and healthier person might not have been injured at all. You have to deal with the consequences of what actually happened due to your actions, not what might or might not have happened under different circumstances.
Another legal principle I've heard about is "Intent follows the bullet." If you shoot at one person but hit another, even entirely by mistake, you are just as guilty of attempted murder or murder itself as if you had hit your intended victim.
In this case, absent being shot in the first place, the victim would not have needed a blood transfusion. And no one can say for certain a blood transfusion would have saved this particular victim. Either way, the proximate cause of death may have been exsanguination, but the manner of death was homicide.