Kang Tae’s struggles made me overlook Sang Tae’s emotions throughout the episodes. Sang Tae almost talked about when he hated Kang Tae the most earlier in the episode to Moon Young after he accepted her as his best friend. That was his secret. That Kang Tae wanted him to die and left him alone in the frozen river before rescuing him. Then, after being resuced, he left Kang Tae alone in the river, which I think is purely  out of survival instinct. But, the memory of being left alone had stayed in Sang Tae’s mind ever since.
In his sketchbook, he drew a picture in which three of them going out on a trip with a camping car. So, he was accepting Moon Young together with him and Kang Tae as a group of three.  So, when he found out about Kang Tae and Moon Young (incorrect facts, through that incompetent lousy caregiver), out of insecurities of being left alone, he made the conclusion on his own that Kang Tae was lying based on his childhood memory.
Which is heartbreaking, because Kang Tae came back again and saved Sang Tae out of the water and has been living with that decision ever since Moon Young saved Kang Tae. And in Kang Tae’s eye in the last scene, I just saw apologetic eyes for Sang Tae remembering all those memories and living with them.
Moon Young knew how and why both of the brother feel and that’s why she didn’t act out on impulse like she always did.  
This is just how I perceived last 5 mins of ep 9 of IONTBO. 
Moon Young, Kang Tae and Sang Tae all needs help psychologically, but that help can only comes from that circle of three, not from outside.

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    Yes to all that! All of them have been keeping that memory with them until today. Sang-tae remembered Gang-tae leaving him behind, not the part that he saved him. Gang-tae puts emphasis on him hesitating to save Hyung instead of the part that he actually endangered his own life to save his brother. Moon-young witnessed all this but she too has her own twisted lenses. It’s insane how we probably don’t even remember things correctly but memories stay with us in accordance to what we felt during that moment.

    The last part of your comment reminded me of Shin Se Kyung’s line from Tree With Deep Roots. “His Majesty and Kang Chae-yoon are people who have fallen ill but the only person who can treat them is each other.”

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      A betterexplanation of what I wanted to say.
      Director Doctor did not interfere and let everything happen and just observed. I think it is for the same purpose.

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        Yessss. Sometimes, we have to just stand and watch. It was release for all of them. They’ve swept that under the rug for a long time thinking meh the other person probably forgot about it. Now that it’s out in the open, they can’t just ignore it. They have to do something about it and that’s a part of healing too. I like the Director. He observes them and let them naturally heal.

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