In short, Guido died because he threw himself out of a window. He was told his father didn't want him anymore which was not true.
In the story, Guido is the child of a peasant family. The narrator rents a broken down cottage from Signora Bondi. He soon discovers a brilliant little boy, Guido who plays with his son. The boy is very smart in music and mostly in math.
Signora Bondi takes up with the boy and babies him and dresses him up like a doll, a play thing. She wants to create something out of him, but she doesn't really care about him. She manipulates the parents into letting the boy live with her because she says she has money and will help him. She manipulates the parents too. The parents have no idea what is really going on with heri son in the home of Signora Bondi. The narrator tells Guido's parents to be weary of her.Over time, S. Bondi has the boy believing that his parents didn't want him. S. Bondi denies him what he really loves, math and dresses him up to suit her taste. She is trying to create an image that she likes, not what is best for the boy.
Eventually he commits suicide.
But it's too late for anyone to save him. This novel was written during the rise of facism and contains many illusions to the futility of individual struggle in the face of a facist dictator.It's interesting to note how gullible the peasants were and how they had no idea what was really going on with their son. This is an illusion to Europe and the world during the rise of Hitler and facism. Everyone in Europe would not believe what Hitler was doing to the Jews. Until it was too late, that is, for many.