Sarcasm is an adjusted stress reaction.

My first thought (on reading this) is "no no no no" because the Not-In-A-Spacesuit!River is already in prison for killing the man she respects most, so she should have remembered the situation and not been surprised by the sudden death of the Doctor. On a second watch of the episode, though, her reactions seems to leave room for that interpretation. Although the idea that she'd shoot at herself is interesting.