So who did Negan kill?
My thought was that Abraham or Aaron would have made the most sense. Picking the strong ones to make an example.
This video of the audio might make a fairly compelling case for someone else. From a rational standpoint this is not the choice I would have thought Negan would make. You don't need to cause any extra hard feelings on top of what you're already doing, right?
http://comicbook.com/2016/04/04/the-walking-dead-finale-audio-may-offer-clues-to-who-died/