The reason why Hannibal is able to ignore Will’s welfare, I think, partly has to do with the fact that Hannibal himself wouldn’t feel protective over his own illness. Hannibal, if he was in Will’s place suffering with encephalitis, wouldn’t care so much about pain and other symptoms as much as he would feel possessive over his future actions. For example if he had a motive, he would be more concerned with accomplishing that motive despite his illness rather than the illness itself. Hannibal is comfortable with the idea of death, and is able to view his physical wellbeing through a practical lens. In a way, his mind is soooo inquisitive and focused that his own body becomes a lab rat in certain contexts. The most important thing to Hannibal that Hannibal possesses is his mind. Same with Will. Hannibal can watch Will suffer with a 105°F fever because he is more invested in Will’s motives, and what Will does to achieve those motives, and how he will go about doing it with his brain on fire. In this way, I think, Hannibal’s extreme artistic and scientific perspectives balance each other.