From now on

Sei looked like a father who was so glad that his son finally chose the right path. He had been waiting for this day for many years. Sei had been searching the world for a cure all this time. He wanted Zaki to live because he thought that this was the only thing he could do for him.

Moreover, to Sei, Zaki was his responsibility. He could never just stand there watching his brother die. That was why he never gave up searching for anything that could cure him or at least lengthen his life. 

However, Zaki had already accepted his fate. He didn't have the desire to fight for his life. He believed that since it was inevitable, there was no use fighting for it.