Both Mion and Rena behaved smartly with Satoshi when he behaved like that. Most of these questions are really explored in the Tsumihoroboshi-hen VN. That’s her tiredness of doing it for 100 years. she just has a bad tendency to give up on the world and her friends and just let it end. Remember, Mion and Rena knew nothing of the syndrome really. But the one who you can really blame is Rika.

Indeed, it was a mistake to hide things from him.

Rena and Mion commited suicide by willingfully let Keiichi beat the hell out of them. Why did they withstand so much pain for no reason? Keiichi was obviously out of his mind for a while, they closely saw Satoshi going down into madness. I remember there's a flashback scene in the anime where Rena holds her arms open as Keiichi proceeds to bludgeon her. If Rena and Mion had enough physical prowess to hold Keiichi down to use the marker how was Keiichi able to knock out both girls with a baseball bat? At least one of them should've ran for their life. Keiichi was obviously screaming for his life, unless the syndrome makes the person act normally outside while they're snapping inside. How could they still go to his house to play a punishment game when things were so out of control, couldn't they see how terrified Keiichi was? They were psychologically abusing him during that last visit. Rena and Mion knew exactly what Keiichi was going trough since Satoshi behaved the same until he snapped.

It was obvious Keiichi was very afraid of them and didn't want to keep in touch. They saw Keiichi growing suspicious of them, and saw him training with a bat for self-defense. They wanted to forget the violent past and still act like nothing happened. How could Rena and Mion be so 'naive' in order to keep in touch with Keiichi? They were indeed hiding things from him.

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