I have always been a nitpicker. I am picking nits, as is my perogative. The double question mark indicated to me excitement. "Who is that guy", within the context of our language, can have two meanings. First, if asked in a neutral way, "Who is that guy, over there in the corner", then you are asking for identity. However, "Who is that guy??" more often than not carries a suggestion of amazement, as I believe we would both stipulate yours did. However, to me, your post has a tinge of admiration. "Cold-blooded", "ice water in his veins", "even military skirmishes don't have [such kill ratios]".
If any of what you had said, him being a good shot, and him doing it calmly is correct, then your post will have been well-placed, if not, in my opinion, well-phrased. That said, recognize that errors in interpretation may occur. If you were indeed not saying your comment in a mood of awe/admiration, then my interpretation was incorrect, and for that, if that, I apologize. That said, were that your intent, I found your post poorly phrased, though your clarification put it back in good order for me.