Harry feels like he has been forgotten about at the Dursleys. His friends don't believe him about the rise of Voldemort, they believe he couldn't share the glory and killed Cedric out of jealousy. The only person Harry knows never lied to him was the only person he was supposed to hate. Dumbledore knew what the Dursleys did to him only he did nothing he would go on and on about blood wards no move was made to protect him. The Dursleys did something that made Harry snap. Blood wards be damned, Harry wanted to kill the muggles but he would do as a message to Dumbledore and the order of the Flaming Chicken.