So many people might be willing to go be a health worker in a poor country where aid workers are commonly (1 in 10,000) raped or killed, even though they would not be willing to be certainly attacked in exchange for 10,000 times the benefits to others.

I agree with your main point, but the thought experiment seems to be based on the false assumption that the risk of being raped or murdered are smaller than 1 in 10K if you stay at home. Wikipedia guesstimates that 1 in 6 women in the US are on the receiving end of attempted rape at some point, so someone who goes to a place with a 1 in 10K chance of being raped or murdered has probably improved their personal safety. To make a better thought experiment, I suppose you have to talk about the marginal increase in rape or murder rate when working in the poor country when compared to staying home, and perhaps you should stick to murder since the rape rate is so high.