Why does Nathan plug the tree in to kill mockingbird?

It appears you may be referring to the character Bob Ewell from "To Kill a Mockingbird" by Harper Lee, not a character named Nathan. Bob Ewell, in the story, is seeking revenge on Atticus Finch for exposing the truth in court. Bob tries to harm Atticus's children, Scout and Jem, by attacking them near a tree. Atticus shoots and kills Bob Ewell to protect his children.