Time travel.
This is unscientific and nobody can tell if it's true or not.
But the Boy travel back in time before the whole Zombie Apocalypse occur.
His Borther is gay and their parents are against their son's relationship with his gangster's boyfriend but in Apocalypse, he is the first--no,the only one who reach out to help their family.
Thanks to him they didn't got eaten when the zombie beseige the capital.
But thanks to their domineering parents and his own brother's selfishness.
The gangster have fall out with him and find a lover who apparently have a space power and has so many great supplies (who knows where he gets them).
In anger, his brother and family didn't follow the Gangster's group despite the gangster promise that even though they broken up he will still protect him.
So they leave with pride and died instantly.
And him?
He is the younger brother.
■■My original work■■