Does not each 'hostage' become increasingly more emotive? Start with a woman, then a young man, then an old blind woman, then a child. Finally Sherlock's friend.

I found out interesting that Sherlock and John had a bit of a tiff when John pointed out that lives, real human lives are at stake, and Sherlock said something about will caring help, then I shall continue to not make that mistake. (paraphrasing).
And yet at the pool, he tells M that People have DIED!  to which jim replies that's what people DO!

So while solving the case, Sherlock understands that lives are at stake, but doesn't let the emotion interfere, while he knows, and CARES about it.

When he says 'People have died' Sherlock is making a statement of fact. He must learn useful things about Moriarty by the extreme reaction he gets from him.

I think the blind lady was killed simply because she started to describe Moriarty's voice. My theory is that Moriarty at that point was already planning to blame all his crimes on Sherlock later. When the old Lady started to talk about his voice, Moriarty realised he had made a mistake when he had talked to her. Being blind, her hearing was probably good enough to be 100% sure that the voice she had heard could not have been Sherlock's. So she had to die.