What does Hurst mean by this sentence? "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."

Brother is narrating the events of this story, but he is narrating these events to his readers a long time after they have occurred. He's a grown man, and he is telling us his memories about Doodle. The sentence in question is an interesting sentence because it breaks the narrative flow just a little bit. We have gotten used to Brother narrating about Doodle and himself as if each of those events is the current present event. Then this sentence comes along, and it reminds us of the fact that the narrator is older and looking back at these events. Additionally, the quote is important because the narrator is sort of attempting to give us a reason and/or explanation as to why he was mean to Doodle. The sentence is trying to explain that Brother loved Doodle dearly; however, with that love came cruelty. Streams carry more than just water, and not all of it is good. Blood is healthy, yet it can become infected and become dangerous. Brother's love is there, but so is his propensity and desire to be mean to his brother. Anybody that has a sibling understands this quite intimately. You love your sibling, but you also love to bug them. A sibling relationship is different than a friend relationship because there can be more cruelty in it. There's not less love. There's love for the sibling and the love of pestering the sibling. This sibling dynamic is common, and it is probably why the narrator openly admits that he has seen it in others. He's also sad about seeing it in others, because he has lived through the pain of seeing that sibling die, and he now wishes he had never acted cruelly toward Doodle.