Vyvyan Basterd
Adventurer
I suspect it's just that they're going over the top with the reaction deliberately in order to highlight that This Is Important, rather than anything else. It's "Doctor Who" - I'm not sure they do 'subtle'.
I think it's a commentary on modern military. With the attitude against military action in the modern era, I could imagine a soldier being sensitive when people thinks all they do is kill people.
I thought it was incredibly insensitive of Clara the first time she joked about killing people. I would never do that to someone in the military. Then, once she already knew he was sensitive about it, she DID IT AGAIN. I wouldn't do that to anyone unless I was intentionally trying to antagonize them.