There is still a very slight possibility that Negan doesn't actually kill anyone. The writers have already deviated from the comic books so they could do it again.
Now hear me out...
Negan doesn't actually say "I'm going to kill one of you". He actual line was "So, now I'm gonna beat the holy hell outta one of you."
So, it's possible that the writers did this on purpose to buy themselves time between seasons, or to test the waters with the audience to see how they would react.
Remember, there is no such thing as "bad publicity". The show has already won because everyone is talking about it. So, if the audience revolts, they will just keep the victim barley alive. And if the audience was OK with the finale, they will let the person die. Either way, the writers win because they can do whatever they want before season 7.
Also, remember that Carol and Morgan meet up with those Horseback dudes earlier in the day...so it's technically possible for them to interrupt Negan's beating before his victim dies.