Just a thought, but human lifespan, at least non-cancer ridden human life span, is believed to be governed by the telomeres in our DNA. They act as 'protective caps,' if you will, that help ensure good copies, but they get 'worn down' by the process. If that 'life sacrifice' was basically inhibiting the function of telomeres in the body, then sacrificing individual cells is what they are already doing. As they use magic, more of their own cells will be killed in order to avoid negative mutations (ie, cancer) until the ability of the body to stop it from happening is overwhelmed.

Not an expert, just fascinated by DNA and, well, the effects of small changes in the code that can causes big issues.