What did Nana do when she realized that her daughter left the kolba? Why?

Nana hangs herself from a tree when she realizes that Mariam has left. Nana could not live with the thought that Mariam has gone, especially to live with the husband that banished her to the Kolba. Nana refuses to be alone in the world that has been so cruel to her, she refuses to see her daughter run to the man who tossed out both of them.