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The way I understood chap 43's highlight, was basically Hangyeol finally realized that Hanyool might actually be the killer in the future, so he's decided to bring his brother down the cliff with him, but Hangyeol didn't have enough strength to really chuck the both of them down the cliff, and Hanyool realized what Hangyeol was trying to do, out of self preservation pushed Hangyeol off him, but unknowingly down the cliff instead, so that was why he looked so shocked when Hangyeol fell and he could only sit there while Taehyuk jumped in to save the person Hanyool thought he was protecting all along.
Pretty sure that's why he is having a breakdown on the last panel, because he realized, he kept Hangyeol around him because he needed to feel wanted and depended on, but if his life was in danger, nothing else really mattered, and that included his only brother I guess.