Sorry but i agree with his brother in this one...
I get he was desperate but thats not normal. He certainly wasnt going to kill himself but that act just to force Yuzu was a very Shima thing to do, including the smirk at the end. This could had been dealt in many different ways. Kind of stained the story a lil bit for me. Ngl, im a lil bit worried for Yuzu now.
oh i very much think he was dead serious. if yuzu had chosen her mother then tamahiko has nothing left to bind him to this world. yuzu is very much everything that matters to him. hope is quite sinister. love is sinister. many just don't get to taste the sinister part. this is because many of us grow up feeling love.
we grow up already knowing its warm embrace. tamahiko did not grow up feeling love, yuzu was his first experience feeling its warm embrace. now that he tasted love returning back to a loveless life is a fate worse than death in his eyes.
this is the sinister side of love. this is the sinister side of hope. if you grow up without it then tasting it is so much sweeter,
but having tasted that sweetness you can never go back.