Not really spoiling, just speculating and explaining my rationale from hints from last season. In my view the person killed is . . . Abraham. What's more, Eugene is a traitor. Hear me out. There are a variety of mysterious factoids I have strung together here. Start with the premise that Eugene is a really smart guy, and has become (as emphasized in last episode of last season) a survivor. He has figured out how powerful the Saviors are and allied with them because he knows they are the real power and best chance for survival. We also know that Eugene is quite capable of deceiving those close to him, for a long time, to achieve his ends. That's how he teamed up with Abraham to begin with as he played his "if you can just get me to DC I can fix this" virologist con.
The most compelling evidence is that The Saviors were always one step ahead of the good guys suggesting they had an insider. Well, guess who was in charge of the map? You guessed it, Eugene set them on every route they took, routes that ended with Saviors in front of them in progressively greater numbers. Did you notice that Eugene added himself to this mission to The Hilltop at the last minute at his own insistence?
Eugene needs to be separated from the main group before any potential large battle so he isn't hurt in a gun battle crossfire, so who comes up with the plan to separate him from the main group? Eugene does. His plan to set the main group off in dark the rest of the way while Eugene alone diverts the bad guys with the RV should work, unless the Saviors have advance notice of it. It doesn't work as apparently The Saviors do have advance notice. Our heroes blunder on foot into a well laid Savior trap that could only exist if the Saviors had advance notice of what they were doing. That notice came from Eugene.
As the party departs from the RV there is a fake tender moment as the party is convinced Eugene's staying with the RV is a noble, self sacrificing gesture almost certain to get him killed. We then have a private moment with Eugene where his emotions are emphasized. He frowns, but then smiles with resolve. At the time we think he is smiling in acceptance of his fate. I think that moment was all about a tinge of regret at what he was doing to his former friends followed by his acceptance that it will be good for him and a bit of pride that his treacherous plan is working perfectly.
Of course, we see Eugene at the Savior ambush point apparently captured by the Saviors. He's a little beat up, but not much. Notably we didn't actually see him get captured by the Saviors. There's a reason for that. He wasn't captured, he just rejoined his allies. He's roughed up a bit as part of the charade because Negan plans to continue to use him as a mole to monitor what goes on in Alexandria.
Notably, that is at least the second time Eugene is captured off screen by The Saviors after arranging for himself to be alone. The first is in the warehouse where Eugene discloses his plan to Abraham to make bullets. Eugene tried to go there alone, because the real purpose was to meet and plan with Saviors, but Abraham insisted on coming. Eugene insults him into leaving once they get there, leaving him alone to be "captured" off screen, isn't that convenient.
Once captured he is used as a faux hostage so he can be reunited with the group as a hero with his loyalty unquestioned and the plans now laid with The Saviors for him to lure the group into their clutches. I would not be surprised if Eugene isn't responsible for Maggie's sudden illness by some sort of poisoning. This forces the group to make the run for The Hilltop that The Saviors seem to know was going to happen. Indeed, the Saviors even seem to have foreknowledge of the spontaneous foray led Daryl, as if they must have a mole working for them in Alexandria. I suggest that they do.
And about the bullet manufacturing, there's no set up that our heroes in Alexandria have any particular shortage. Yes, Eugene notes that they are "finite" but it seems awful forward planning for them running from one crisis to another. I submit that the group short on bullets is The Saviors and Eugene has offered that service to them, that the people of Alexandria will be providing the labor for.
So why is the victim Abraham? Because Abraham nearly beat Eugene to death and Eugene has not forgotten it. His deal with Negan includes Negan taking out Abraham with Lucille. Eugene gets the security of The Saviors and revenge on Abraham in one tight neat little package. He is, after all, a really smart guy.
Anyway, that's my little pet theory. Fire away.