Was the narrator, Brother, guilty of Doodle's death? This is a difficult question because Doodle also has bias in the story being so vulnerable...

According to own character description, Doodle died of a heart attack.  The tree did not kill him, but your main question still stands.  Is Brother guilty of killing his brother?   Or is he guilty of playing a part? To be really cold, no, Brother didn't kill Doodle.  Genetics killed Doodle.  Doodle was born both mentally and physically handicapped.  The defect in his heart was likely always there, and his heart was waiting to...

According to own character description, Doodle died of a heart attack.  The tree did not kill him, but your main question still stands.  Is Brother guilty of killing his brother?   Or is he guilty of playing a part? To be really cold, no, Brother didn't kill Doodle.  Genetics killed Doodle.  Doodle was born both mentally and physically handicapped.  The defect in his heart was likely always there, and his heart was waiting to fail.  It just happened to fail while Doodle was running after Brother.  

It is possible to think that Brother deserves some of the blame though.  Brother grew impatient with Doodle and ran off ahead of him.  Doodle struggled to keep up and overexerted himself trying to stay with Brother.  The extra strain on his heart could have been the trigger that ultimately caused Doodle's heart attack.  Had Brother gone at Doodle's pace, the heart attack might not have happened at that moment.  I still don't think Brother is guilty of Doodle's death.  He probably feels guilty for having left him, but I don't think the narrator should feel guilty for causing the death.