Season 6 finale spoilers...
It wasn't Carl or Rick based on what Neegan said. I don't
think
it was Maggie or Darryl, considering whoever it was got back up after the first hit (they both looked rather awful). The background behind Neegan was trees, but I suppose that could have been to throw us off. I think it might have been Abraham, considering he pretty much volunteered at one point by raising up on his knees. Neegan also had a bit of reaction to Abraham doing that, moreso (imo) than his reaction to Glenn or Rick getting agitated when Maggie/Carl were threatened. We should have seen some hair moving around from the victim's standpoint if it had been anyone with long-ish hair like Michonne, Rosita, Darryl, or Glenn. Sasha had her hair tied back, so it could have been her. I could see it being Eugene too, since bad guys especially don't like tattle-tales.
On to other questions though...
What worries me most though, is whether there was only
one
person who died. Maggie looked like she was on death's door even before someone got beaten to death, and Idk if the baby would survive even if Neegan allowed the Grimes Gang to try to save it. Carl looked like he wanted to take all the Saviors on by himself. There are a lot of the others that probably wouldn't take it lying down either, especially depending on whom Neegan chose to kill. Even someone without intimate attachments might think that all their lives were worth risking in taking a chance to kill Neegan with a hidden dagger. Neegan seems to be the head of the snake, as it were, so there's the possibility that killing him might save a lot of other people, even if it didn't save those members of the Grime's Gang who were present. I don't think I heard any gunshots, but Dwight still had Darryl's crossbow and there were other 'quiet' weapons among the Saviors.
Hum, out of room in this post