It is for me in canon, definitely, and anywhere he's got a military background (and I think that's why it's such a common thread to AUs, even complete AUs).

But in something like my diner fic, I don't know that John would've necessarily developed the dark edge, not in that way. I tend to think it's a combination of being able to make (and having to make) the hard choices and having the training to do so. Take out the truly hard choices and the training, and you... well, make him a little more average, I guess. At least in my reading of him. Hm. Will have to think on this more - something is niggling at me about it.