Why the blue key as proof of mission accomplished?
Why would the hitman leave a key behind the alley when the deed was done? Diane would have read it in the papers if Camilla was murdered. The key doesn´t proof the hitman did it, because he could have left it there without being the one that had killed Camilla. Wel, the point is: the key doesn´t serve any purpose in this regard.
Moreover: the key is blue, just like the blue box. It could be that Diane modeled the box after this blue key that signifies this horrible deed, but if she did than why do we see this elderly couple coming out of it and not (dead) Camilla?
Last question: Why does Diane bring the key home? wasn´t it enough just to see the key in the alley? When she brings the key home she implicates herself more.