@Asalsio
Much like what it was answered, he has a Persecutory delusions, a condition where he or she is convinced someone's after them. In his case, the professor's condition's worse due to his old age where he forgot the things he himself fabricated, causing him to have fake memories.
Touma blames himself since due to the keywords, the professor keeps remembering back John and even forgets every answer Touma has told him until he gets the answer he wants.
I won't say pitiful but you can't blame someone who's at that age. Had Touma not stop him, the professor might be arrested or sent to a therapist to treat his disorder (Which I believe he doesn't know.)