I must disagree on the "fact" about the dementors. On page chapter one of Harry Potter and the Half Blood Prince, Fudge states "That's right. And they're [the dementors] breeding. That's what's causing all the mist.
In Petunia's mind, "that awful boy" killed her sister. She didn't hate him because he was a wizard but because in her mind he took from her her only sister. Petunia loved Lily, no matter what she feigned.