There’s a difference between being misunderstood and being misguided. Draco and his Slytherin friends are misinformed and misguided by their parents, they are not misunderstood. They were awful, they did horrible things, they weren’t secretly non-bigoted underneath. They can learn from their mistakes and grow up to be better people, that still doesn’t mean their past actions are excusable or that Harry was prejudiced to dislike them. They earned his dislike, they can earn his/others forgiveness too.