They engineered a psycopath to kill you
River Song + The Day of The Doctor
Doctor Who Series 4: Forest of the Dead
Hey Anon! For me, at least, I’m going to assume that if River knows the Doctor’s real name, she also knows his biggest secret - the one that he will take to his grave in Trenzalore. The Name Of The Doctor established quite strongly that John Hurt is that secret - and it seems likely that he’s an incarnation/regeneration of the Doctor that, shall we say, the Doctor does not consider canon. So what better way to make sure Ten knows how much he will come to trust her by whispering about John Hurt in his ear? (That sounds slightly dodgy, lol.) I think the faint horror with which David plays his reaction is just perfect - it can mean so much and now it does.
Doctor, one day I’m going to be someone that you trust completely, but I can’t wait for you to find that out. So I’m going to prove it to you, and I’m sorry. I’m really very sorry.