Jupiter is dead, but the real battle has only just started: Sailor Moon, the princess, no longer wants to move on. From the way I see it, this attitude stems from her unwillingness to accept death in battle. While that she has witnessed personal loss (Naru having to watch Nephrite die) and has experienced some serious stuff herself (Mamoru’s kidnappings, finding out about her past life, …); she is still sheltered compared to the other Senshi, and she is not at all near ready to witness such a brutal death of her dear friend. On top of that, Sailor Moon is also struggling with guilt: Jupiter dies to protect her as well as her ginzhuishou. In her mind, she (and in extension the Silver Crystal) is probably as responsible for Jupiter’s death as much as those shocks and vines from the DD girls.
Those thoughts swirl around in Sailor Moon’s mind and trap her in a dangerous mentality. She has convinced herself to give up fighting, and is running towards the enemy to make all the Senshi surrender. Neither Venus’ indignation nor Mars’ scolding have any effects on Sailor Moon, and physically restraining her is not good enough for the situation. So, what should the Senshi do?
Sailor Mercury, normally the most gentle of all the girls, steps up and slaps Sailor Moon straight in the face. She does not enjoy the deal any more than any of the audience, and withdraws her arm into a contrite gesture almost immediately. But it is not an act on impulse, as evidenced by the look on her face right before the slap happens. Mercury has calculated all the moves possible and decides that a slap, may not be the best way to treat a comrade, but is the fastest way in this situation. Mercury may not be able to completely convince Sailor Moon, but at least she’s pulled out of her charged emotional state to stop running to Queen Beryl, giving them more time to deal with the approaching DD girls.
Then Mercury delivers one of the greatest acts of the whole series: She offers to sacrifice herself. Considering that the whole Sailor Moon series is full of great sacrifices (even in this very episode), somehow Mercury’s choice here sticks with me the most. Maybe it’s because many sacrifices in Sailor Moon are instincts lead by the heat of the moment and usually supported by the capability of the person doing the act, while Mercury’s action is the opposite of those things: she sacrifices because she does not have (offensive) powers.
We also need to admire how smooth Mercury lies to Sailor Moon here: anyone with a bit of sense knows that Mercury won’t survive an ordeal with the DD girls, and Mercury is the person most aware of that, yet she easily lies and smiles at Sailor Moon as if the upcoming confrontation is just a test to beat. If Mercury displayed even a smallest bit of the knowledge of her certain death, Sailor Moon would be sure to be able to pick up and stay behind, maybe even try again to surrender; so all Mercury shows is just confidence of a guaranteed success.