After rebirth, he became Harry Potter
He likes beautiful things, the Malfoy family's hair and Severus' eyes. In his eyes, beautiful things should be his.
He admires Slytherin, but he is determined to become a Gryffindor. He wants to change the atmosphere of Gryffindor and create a more noble academy than Slytherin.
He didn't want to be controlled by Dumbledore anymore, so how about joining forces with Voldemort? It was an era of turmoil anyway, so it would be better to add to the chaos...