God, I can't believe I'm defending Kim, but someone should wear a fucking hero cape.
Kim left him to save Mc's life. She could have told Mc but Kim didn't want to gamble with Mc's life. What she did was not right, but I care about her intentions here and she definitely had good intentions.
She didn't cheat on Mc or let another man into her life. Now she is fighting to win back the man she fell in love with.
Despair and fear can make a person do anything, especially for the ones they love.
If the developer wants to take some of the hate away from Kim, they should include Kim's point of view in the game. It might be useful to show what she went through during her time away from the MC - regret, fear, desperation, the struggle to get back to the MC, etc.