Well, The whole game is told from Jack's perspective. And he doesn't think he is an asshole. He thinks he is a good and virtuous dude. But he says it himself at the start of the game: around him, most people say that he is an asshole. He objectively is. The game, however, is subjectively told. He has done more than just believing the rumours. I give a few hints later in the game.
That being said I have to agree that the beginning of the game is a bit weak and I have some rework to do there. That discussion with Cassie on Sunday morning is on my list of things to rewrite. It clearly needs more context and more depth to be fully understandable.