OMG! Be still my heart! Those fireworks are similar to those Susu made for him 500 years ago but they are not identical; Tantai Jin has drawn them for her using the sketch she taught him but he’s hiding it from her (when he waves his hand his spell doesn’t emit the red colour of the Devil God power but golden glow, just like her sketch)!
Tantai Jin’s most powerful and magical ability isn’t absorbing energy from demons but finding happiness in the smallest things others might consider trivial and turning them into bright rays of light which keep the darkness inside him at bay. The mere thought that Susu once believed him capable of being good and that he didn’t fail her, fulfiled her wish and is, in fact, good, even though she remains oblivious about it, is enough to make him bliss out and giddy, experiencing unadulterated happiness and joy. His proud smile and the question “does it look pretty?”, like he is waiting for her to review his spell, the only way he allows himself to show the truth.
He managed to draw the sketch already 500 years ago but actually kept it a secret from her ever since, probably the only part of himself he has never revealed to her and has been withholding from her, but is secretly showing to her now by creating the spell again even though she doesn’t know it. For Tantai Jin, this is another scrap of happiness he tucks and guards inside the deepest recesses of the heart he gave Susu centuries ago.
While he tries to make her hate him so she could kill him when the time comes, he doesn’t want her to resent him in his “Bo’re life”, this refuge inside his heart where he escapes when his demons and loneliness plague him.
This is a dream, a fleeting moment of elusive happiness he decides to steal for himself before he sacrifices his life for all living beings in the universe, to give him strength to do what he must and have something to remember when he turns into nothingness. He wants to do something selfish one last time and take something for himself, instead of constantly giving while receiving nothing in return - one final moment in which Susu loves him and belongs only to him, and he only to her.
And secretly, Tantai Jin wants to leave Susu a piece of himself behind, a proof that he and his love for her were real and existed, no matter how insignificant compared to the cosmic struggle between good and evil, hoping she would remember him once she learns the truth even when no one else would, especially when it was his love for her which ultimately defeated the darkness inside him and transformed him into the man he is today.
He doesn’t fear death but being forgotten by Su because what is oblivion and being forgotten other than second and ultimate death, being abandoned and unloved one final time.
Tantai Jin’s final wish is for his love to survive him, defeat and overcome death; he desperately craves to live inside her forever, this way he can become immortal too and be with her forever.