Negan probably needs to suffer
He probably absolutely has to suffer at the hands of Rick. The thing is, how far can Rick and the show go with Rick killing Negan? I'm not talking about the amount of blood or gore is involved, I'm talking about from a moral and human perspective. Would it be okay for Rick to turn into an absolute monster while killing him? Can Rick come back from that? How far over the line can you turn into animal in that world and still be a respected leader?