We, The Infected

In the novel Red Dragon Will Graham is the one in charge of the case of The Tooth Fairy, Graham has this unique gift to understand the criminally insane, Graham by the end of the story realizes that he could perfectly be one psychopath, as well as the people he hunts, Graham posses all the characteristics of a true criminal, although he is the "hero" of the story.
Eli has killed numerous people but he never gets satisfaction in doing so, Graham in the other hand can enter in the mindset of the killer, he knows what the Red Dragon (Francis Dolarhyde) thinks, feels, desires, does this make Graham a far more unstable person than Eli?

Dear little Eli may well be inured to murdering people, but that doesn't imply that she has no feelings for them.

In the novel rendition, Will never kills anyone except for Hobbs in the backstory, and overall remains quite mentally sound despite his unique disposition and the pressure placed on him. Elias kills regularly, whether it be directly or indirectly, but also remains quite hardy of mind.

So if your question is focused on their mental fragility, both of them are exceptionally stable given their trials but Will comes out on top - why? Because Elias, if sufficiently provoked, has demonstrated the capacity for the violent murder of children. If it’s a question of lethality alone, then Elias takes it easily. At the end of the day he’s the one who requires someone to die several times a month to live.