Context: Aang ("A") is a classic Batman's Rule (never kill) hero, as a result of his upbringing in Air Nomad culture. It appears to him that he must kill someone in order to save the world. He is the only one who can do it, because he's currently the one and only avatar. Yangchen ("Y") is the last avatar to have also been an Air Nomad, and has probably faced similar dilemmas in the past. Aang can communicate with her spirit, but she's dead and can't do things directly anymore.

The story would have been better if Aang had listened to her advice, in my opinion.