I think if Sasuke ever had killed Naruto. Naruto would have definitely kept on smiling. I also think that Naruto smiling after Sasuke killed him would either make Sasuke turns his ways around, or it would fuel his anger even more.
Amaterasu - the flame that doesn't die until it burns everything - it literally means hatred and sorrow. So, the caster of this spell is in essence cursing you to death by hatred and sorrow. Such a terrible way to die.