If You force the outcome to be soly on Your decision alone and if Your decision is clear, free and consistent with a specific philosophy, then You must be judge acc. to this philosophy.

Which philosophy is valid in a Least Convenient Possible World?

If everything I do to "humanely" help the patients without commiting murder to the strange ris futile AND and if none of the patients would be willing to do a self-sacrifice to save the others AND if the sole and only decision to this situation would lie on me, then (my clearly idealized) I would teach the donor all the neccesary skills to kill and harvest me to save the others.

If not even that is allowed, then yes - a utalitarianistic murder of the stranger would be legit, beacuse You have trully checked for all options, to freely and through selff-sacrifice try to save the patiens - without success.

Only when You eliminate all humane options can You turn to the "inhumane" (I use thet term loosely - in this case, at the end, it was a humane sollution) - if that brings out more utility/less global suffering/more global pleasure and freedom.

But again - this is not a realistic option. Realistically it is almost certain that a humane approach would become viable before that.