I saw a movie called House at the end of the street. I don't get it?

It's called psychology. People don't get over trauma. Not to mention, it's a movie.

Ryan is already "messed up" and could never lead a normal life. When Ryan is placed in a psychiatric ward [at the end of the movie], a flashback shows young Ryan about to blow out birthday candles. His mother calls him "Carrie Anne" and when Ryan protests that his name is Ryan, not Carrie Anne, she slaps him violently; it is revealed that his parents forced him to dress and act like Carrie Anne after she died and most likely abused Ryan when he refused to go along with their fantasy, thus setting Ryan on his troubled path.

After years of abuse, identity confusion, kidnapping young girls, and living in a fantasy that these girl(s) are replacements for his dead sister, Ryan was in a downward spiral. Killing his parents did little to nothing to undo the irreversible psychological damage this boy had endured.