It would be disturbing if Sherlock didn't react to the news of the man's death. It is probably much more disturbing that he reacts with a faint smile.

"His mistake. I was right about the terms of his employment. He was hired to kill Tony; whether or not I caught a bullet along the way was explicitly mentioned to be of no consequence."

The casual way he delivers this information might be down to teenage bravado, Stark arrogance, or the fact that he's Sherlock fucking Holmes. Probably all of the above.