2. Peter Pettigrew
Does it get much worse than Peter Pettigrew in Harry Potter? Not really, and that's why Peter comes in second on the list of the most disliked characters in the series. Played by Timothy Spall in the movies, Peter Pettigrew is one of the more interesting Harry Potter characters. He's obviously friends with James Potter, Sirius Black, and Remus Lupin. He's a member of the Order of the Phoenix. And, for most of the first two books, we believe him to be dead.
In reality, Peter is the catalyst in the death of James and Lily, Harry's parents. He's responsible for their death because he was picked to be their secret keeper, and then he revealed their location to Lord Voldemort. We all know what happened next. Sirius knew what happened, and he confronted Peter, who then killed a bunch of muggles before living the next 12 years as the Weasley's pet rat, Scabbers. All of this is revealed in Harry Potter and the Prisoner of Azkaban.
Despite the first part of that history, Peter gets so much worse. Not only is he responsible for really terrible acts throughout the last four books, he's also one of the biggest cowards in the whole story. Nobody has any time for a person like that, and it's eventually what leads to his doom. I guess he still feels some remorse for what happened to Harry and his parents, and remorse is good, but it's the lack of action that makes him in that remorse that makes him truly despicable in the eyes of many Harry Potter readers and watchers.