After Dagny Taggart found John Galt's motor, she tried to find a scientist who could reconstruct it. None of the people she interviewed worked out.
She had tried to find a scientist able to attempt the reconstruction of the motor. She had interviewed the men recommended to her as the best in their field... The decision to speak to Dr. Robert Stadler had been her last recourse.
She had forced herself to call him, against the resistance of some immovable point within her that felt like brakes slammed tight. She had argued against herself. She had thought: I deal with men like Jim and Orren Boyle - his guilt is less than theirs - why can't I speak to him? She had found no answer, only a stubborn sense of reluctance, only the feeling that of all the men on earth, Dr. Robert Stadler was the one she must not call.
Hank Rearden later commented that Dr. Stadler wanted Dagney to help him pretend that he was still the great Dr. Robert Stadler, and that she shouldn't have seen him. (John Galt later identifies this as "the sanction of the victim"). Also, while Jim Taggart is the primary villain in the book, Robert Stadler represents a wholly different kind of evil - the man who knew better.
So, why was she so reluctant to see Stadler? Did she realize that this was giving him the sanction of the victim, or did she realize at some conscious level that his guilt was arguably greater than Jim's?