If I had come about through the unwilling merger of two people, and my death could restore those people, it's probably ethical to kill me to make it happen.

I don't think it's necessarily reasonable to call the two component people dead either. Death is a not a particularly well defined term, but we don't tend to apply it to people who might get better.

Why don't we just harvest your organs and give them to people we deem more useful, ya know?

The knowledge that you live in a society where you could be legally killed at any point for the greater good, and the resultant fear and uncertainty probably would cause more harm overall than doing so could actually alleviate.