One of the sad aspects of this story is the narrator's sense of pride as he struggles to cope with having a brother like Doodle. This of course forces him to try and make Doodle more socially acceptable, so that the narrator himself would not be made fun of or feel bad for having a brother who wasn't able to do the things that other children his age were able to do. We see this when he decides to teach Doodle to walk:
When Doodle was five years old, I was embarrassed at having a brother of that age who couldn't walk, so I set out to teach him.
Note that the narrator does not teach Doodle for his own good--it is out of his own sense of embarrassment. This is something that the narrator himself later admits when he and Doodle reveal Doodle's ability to walk:
They did not know that I did it for myself; that pride, whose slave I was, spoke to me louder than all of their voices; and that Doodle walked only because I was ashamed of having a crippled brother.
Note how the narrator admits and recognises that he is the "slave of pride." However, spurred on by this success, he comes up with a training programme to teach Doodle how to "run, to swim, to climb trees, and to fight" so that Doodle will be ready for school. However, after the scarlet ibis incident, Doodle and his brother are forced to realise that Doodle has failed in the programme his brother has created for him:
He had failed and we both knew it, so we started back home, racing the storm. We never spoke (what are the words that can solder cracked pride?), but I knew he was watching me, watching for a sign of mercy.
Unfortunately, it is the narrator's failure to give a sign of mercy to Doodle because of his own shame that leads him to desert Doodle to his death. The narrator's pride has been cracked, and he recognises that no words are able to "solder" that crack together again.
The tragedy of this story is that the narrator is unable to accept his brother for the unique and special individual that he is and feels that he needs to transform him into a more socially acceptable individual because of his own sense of shame at his brother. It is this that leads to Doodle's death.