If the choice was between certain death and an experimental drug that might or might not work, I'd certainly choose the latter no matter how unsafe it could be.
When you've already dedicated yourself by traveling across the world to help people fight a highly-contagious disease, it makes sense that you'd be first in line to test a serum (which, while experimental, uses a method that has had great success in the past).
In this case this was a phase I trial. For those unfamiliar with trial design, phase I is not meant to assess the efficacy of the drug but just its safety in humans. It was provided at 1/500th of the maximum safe dose determined in macaques.