Okay, there are issues with the books, sure. Structural issues that cause flashes of "does not compute" in canon whores' brains. Clumsy phrasing. Adverbitis. Etc. But character arcs being "incorrect"? I'm going so speak very slowly now:
J.K. Rowling's characters were created by J.K. Rowling. She may not choose the path we would like for these characters, but that does not mean she is wrong. They are her characters, and therefore it is her undisputable right to do exactly as she wishes with them, even if the resultant character arcs are not compliant with the "rules of great literature". Her characters cannot possibly act out of character, even if they are doing things that they have never done before. She created them, and if she wants to make Harry cast Unforgivables without a second thought, this does not make Harry OOC. It adds a surprising dimension to the characterisation. However, it is not out of chracter. Because he is her character. JKR knows her characters best. If you seriously think otherwise, get HELP. It's fine to prefer your own or a fanficcer's characterisation to canon. Nothing wrong with having a preference. But seriously claiming that a fanbrat understands JKR's characters better than their creator? GET HELP.
I absolutely cannot believe that we have to go through this again after the HBP whining. If you were expecting "great literature" from J.K. Rowling, I'm going to have to question your ability to discern great literature, because the first book really should've been your first clue. That you hung on for years only to scream that JKR betrayed literature with her characterisation choices says a lot more about you than it does about Joanne Kathleen Rowling.
No. Snape, Draco, Colin Creevey, or Tom Riddle should NOT have been the hero of the Harry Potter series. NO. Harry Potter is the hero of the Harry Potter series, he always has been, and just because you're too fucking over-invested IN A FICTIONAL CHARACTER to understand that J.K. Rowling was telling Harry's story it does NOT mean that those of us who loved the book are literary Neanderthals who wouldn't know a great story if it bit us on the nose. GTFO my fandom and go read some Ayn Rand, you pretentious douchemuffins. :|