I think Rei is having a vision of an actual event. Premonitions in this series are always drawn with vague, shadowy, mist covered figures. If it were a premonition of Chibiusa's being killed, the little girl would not have been so distinctly drawn.
^ That's like a lil girl's often-seen dream coming true (albeit briefly) - getting dad all by herself, replacing her mom. Hence the manga kinda "realizes" such dream (on paper), which can never be realized in reality.