or it could be the same reason malfoy doesnt like muggle borns....his parents brainwashed him...no ones born arrogant, stupid, selfish, jealous and so on..the pple they grow up around make them that way so bcuz petunia and vernon hate harry that made dudley hate him too...just my theory- but i also think hes jealous harry can beat pple up with a twig...lol i never got the wand thingi....