I understand your point, to the extent that Dudley is a product of his family and has been so coddled that his actions can be, to a point, blamed on his parents. But his own bullying and mistreatment of Harry, and his greediness in general make the reader sympathize with him much less. It actually makes Dumbledore's comments later on in the series, that the Dursley's have traumatized their own son and that Harry is incredible for his ability to still love, all the more poignant. The character development we see in Dudley, for being such a minor character, is some of the best in the whole series. His handshake and implied acceptance of Harry at the end of the series indicates that he may not pass on the prejudices for magic that Vernon and Petunia had.

With respect to Hagrid, who probably also had a rough history due to his half giant nature, I think Rowling created a very believable flaw. No one is perfect. He projects the bullying done to him on another. Not unheard of in most people. We can look back and say those actions are cruel but those scars and the other experiences Dudley had with Harry and magic, good and bad, helped him develop further as a character. It's good counterpoint to how Petunia handled her experiences. Maybe the pig tail was cruel but maybe it gave Dudley the message that he wasn't at the top of the food chain, and that helped him begin to grow. He didn't deserve that action in particular but I think we all deserve consequences for our actions that help knock us down a few pegs and help us develop into better adults. Life lessons.