Petunia could not stand her sister maybe out of jealousy because in HP#1 she said she was the only one who really saw her sister for what she really was "a freak." Could I be wrong in saying, that if you are jealous of anyone most likely you would despise anyone in contact with that person. I think she was calling James an awful boy, we see what an awful boy he was in Snape's Worse Memory. Dumbledore trusts many people who should be trusted but people do make mistakes, I believe even the most powerful wizard could make them too. Was Dumbledore is wrong for trusting him? I knw that Snape was the one who went through with finishing what Draco could not do. But being that it was either him or Dumbledore, I feel he had no other choice. People have said if they were in a situation where it was kill or be killed - they would not want to die of course then being forced to kill someone.