You know what's the worst thing Gendo can say to Shinji? It's not anything hurtful or apathetic to him and it barely comes before a threat he can think to Shinji's loved ones: Tell him, detail by detail, thread by thread, that he is no different than him, that if anything happened to Asuka like it did to Yui Shinji would have taken the same road as himself without giving a damn of how many people he backstabbed in the process if it meant getting Asuka back. And the scary thing is that he's partially right because our heroes are too emotionally dependent on each other that such an scenario could cross their minds in their darkest moments.