Shinji is a horrible person who ends up totally broken due to his inability to even try and confront any of his demons.

Or he's a very traumatized child who has been thrust into an extremely unhealthy environment, and manipulated every step of the way, and that eventually breaks him. I'm not saying he's a great person, but it's pretty clear that he never stood a chance from the get-go. He still chose to do what he did in that scene, but it's not done in isolation with everything else he's been through.