Why is Moriarty so obsessed with Sherlock?
His obsession with Sherlock Holmes was rooted in how, as mentioned above, he’s the only one at his same level of genius, thus representing a challenge to him: he made up all types of crimes and riddles, and then sat back to watch how Sherlock solved them.
What mental illness does Moriarty have?
Moriarty’s condition would be described in modern psychiatric terms as: psychopathic/severe personality disorder (more on this later).