From the way the scene is shot,
And I don't know if it matter
Sasha, Aaron, Carl or Eugene.
Are all to the left of Rick.
from their point of view the rv is not in their line of sight and the truck is parked the thier left with the lights on negan.
If you watch when negan does the deed the rv is not visible from the victims line of sight even when the camera pans up after the hit.
Also one thing that sorta stood out to me was when negan told Carl he could cry and then right before he hit whomever he said you can cry.
Also he says if anybody tries anything to rip the boys eye out and "then" we will start.
All this leads me to belive It will be Carl.
I think they are going for a shocking surprise like glen was in the comics.
Now I know Carl dieing affects the story vs the comics but after reading the comment from gimple below, it sound like someone else is going to take Carl's place.
Then this comment here from gimple confirms it for me
Following the finale, ComicBook.com had the chance to ask The Walking Deadshowrunner Scott Gimple if Carl and Negan's relationship would be brought to our television screens next year.
"Yes. A thousand times, yes," Gimple said confidently. "But in a very weird... I'll say this - it may be very, very, very brief but we will absolutely see them have a pretty intense moment. There's gonna be a bit of remixing. There might be another character involved that takes some of it. I'm kind of tripping on some aspects of it that I don't want to reveal but that relationship will be absolutely shown."