Snape. From the beginning, the slime-ball has hated Harry's guts. Before even meeting Harry, he was determined to despise him. Uncalled for, you may think. Harry certainly did. But, after much feud, in the seventh and final book, it is revealed that Snape has reason behind his hatred. He was in love with Harry's mom, Lily Evans. He was anguished to learn that his misinformation caused her death, and took his anger out on Harry. This may not be justified, but he had his reasons. It was also revealed that he did not kill Dumbledore out of heartlessness, but because Dumbledore had begged him to, after Snape had made an unbreakable vow to do so. So all in all, even if his actions were a little awry, he had good intentions all along.