Is the narrator in some way responsible for Doodle's death in The Scarlet Ibis? Is his emotion at very end sorrow, guilt, or something else?

The narrator of "The Scarlet Ibis" is telling the story from many years after the events took place. The retelling then is colored by years of consideration, and guilt is definitely apparent as the narrator gives the details of his life with Doodle. At one point early in the story the narrator says,

There is within me (and with sadness I have watched it in others) a knot of cruelty borne by the stream of...

The narrator of "The Scarlet Ibis" is telling the story from many years after the events took place. The retelling then is colored by years of consideration, and guilt is definitely apparent as the narrator gives the details of his life with Doodle. At one point early in the story the narrator says,

There is within me (and with sadness I have watched it in others) a knot of cruelty borne by the stream of love, much as our blood sometimes bears the seed of our destruction, and at times I was mean to Doodle. 

He admits to sometimes being cruel in his treatment of his younger brother. He was often embarrassed by having a crippled brother. The reader, however, cannot help but see that the narrator also loves his brother. Unfortunately, he lets his expectations of having a normal brother get the best of him. After teaching Doodle to walk the narrator admits he did it for himself:

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. 

After Doodle walks, the narrator becomes emboldened and sets out on even more rigorous training for his brother. When that training doesn't work out, because Doodle is simply not strong enough, the narrator loses his temper and runs away, leaving his Doodle in a rainstorm. As Doodle tries to catch up his body breaks down, and when the narrator finds him he has been bleeding from internal injuries. The narrator says,

I began to weep, and the tear-blurred vision in red before me looked very familiar. "Doodle!" I screamed above the pounding storm and threw my body to the earth above his. For a long time, it seemed forever, I lay there crying, sheltering my fallen scarlet ibis from the heresy of rain.

While the brother is partly responsible for his brother's death (Doodle's disability also played a role), the final lines of the story reveal that he is feeling great sorrow over the death. Thus, guilt and sorrow are definitely emotions which could be attributed to the narrator at the end of the story.