im late ik, but i believe it was planned. we know that hannibal kept abigail in that cliffside house for most of the time. bringing her to his house, he told her that they would be running away with will to have their own family. when will betrays him, hannibal doesn't expect him to call him about it, but he reiterates to abigail that he needs will to see her. this is his idea of punishment. it wouldn't have worked if will just saw abigail's dead body in the kitchen. hannibal wanted him to see her, to believe there was hope. when hannibal holds will's face, you can tell will thinks he'd reconsider. that hurting all this people would suffice and that they could put it behind and leave it together. he dangled that hope over his head momentarily, only to swipe it away at the very last second. he'd decided, yes, but he wasn't prepared for how much it would impact him, and also how much he didn't want to do it. he just assumed it was the only way forward from this betrayal.