I think the mood of this excerpt is mournful and disbelieving.
Mournful because somebody died and disbelieving because the death was unexpected. She had only been sick a few days and her condition immediately worsened resulting to her death.
It is very ironic that the major, who is more likely to die young because of his career is the one left behind. It is also very sad because the life of the wife was cut short, she was not able to enjoy being married, having kids, and growing old with her husband.
