Pride is a bad thing in "The Scarlet Ibis" because it causes Brother to be ashamed of Doodle. Doodle is born sickly and weak. Everybody thinks that Doodle will die. Even when Doodle does not die within the first few weeks of his life, the family is convinced that he will always be weak. There is even mention that he might not have a fully functional brain.
He might not, she sobbed, even be "all there."
Brother admits that he loves to run, jump, swim, etc., and it bothers him that his brother will never be able to do those things with him. Brother is too proud, at first, to love Doodle as he is. Because of that pride, Brother decides to smother Doodle to death.
It was bad enough having an invalid brother, but having one who possibly was not all there was unbearable, so began to make plans to kill him by smothering him with a pillow.
About halfway through the story, Brother admits the following about his pride:
They did not know that I did it for myself; that pride, whose slave I was, spoke to me louder than all their voices, and that Doodle walked only because I was ashamed of having a crippled brother. Within a few months, Doodle had learned to walk well and his go-cart was put up in the barn loft (it is still there) beside his little mahogany coffin.
Brother mentions it like his pride was a bad thing. And maybe perhaps his pride and shame first motivated Brother for selfish reasons, but Brother's pride in Doodle's accomplishments is a very good thing.
Finally one day, after many weeks of practicing, he stood alone for a few seconds. When he fell, I grabbed him in my arms and hugged him, our laughter pealing through the swamp like a ringing bell. Now we know it could be done. Hope no longer hid in the dark palmetto thicket but perched like a cardinal in the lacy toothbrush tree, brilliantly visible. "Yes, yes," I cried, and he cried it too, and the grass beneath us was soft and the smell of the swamp was sweet.
When Doodle first learns to stand, Brother is extremely proud of Doodle for doing it. Brother is also proud of himself for being the teacher. Together, their pride continually motivates them to work together more and learn more skills. That pride propels Brother and Doodle to accomplish great things together.