The fact that Will is exposed to Hannibal’s atrocities i.e provoking Mason to kill Margot and Will’s fetus, making Mason eat his own face, finding Alana discarded in a pool of glass at Hannibal’s doorstep , murdering Abigail, finding Anthony’s heavily mutilated body, and of course, he can’t help but want to eradicate him and tries to stab him, but he also can’t help but be seduced by Hannibal’s ideology. To him it’s like hunting a lion or a wild carnivore. You would be insane to justify murdering it for it’s innate violence, but it has to be done for his own safety and sanity. To survive Hannibal, Will has to kill him. At the same time, his heart accepts him naturally. That is his character struggle. He has tamed Hannibal and is tamed by him. The thing he loves most threatens everything else he holds dear, including himself, so he tries to die with him.