into this situation. Living through hell, fighting for humanity, defeating the Honkai. Thats all he has now, even if he never truly cared for it. It became his destiny and purpose. Kevins humanity never died, its been there by his side all along. Perfectly seen in Ch31 as well.
He delayed the inevitable, in hopes there might be a different solution. But this way, Kevin paid his respect to his best friend Su. In the end Kevin - even in Sus eyes - never changed. He has always been the soft goofy boy. Its the circumstances around him that forced him -
Kevin respecting Sus wish, avoiding the final blow to save his best friend. Throwing him out of Sumeru, in hopes he can be succesful with project Valuka. Meanwhile Kevin sacrificed himself &is trapped for god knows how long in the sea of Quanta. Kevin never wanted to start STIGMA