It's Glenn simply because of what the writer's did earlier in the season.
First, they try to prove to the audience that Glenn has plot immunity with the whole dumpster cliffhanger, which they even emphasized by prolonging it for several episodes. "Look audiences, there's no way we're going to let Glenn die, even in a no-way-to-win scenario! And in the exact same half of the season that everyone already knows he's going to die thanks to Lucille!"
Second, they killed off Denise with the crossbow bolt meant for Abraham, which was their (feeble) attempt to prove to audiences that they're not afraid to deviate from the comics. "Look, see, we're totally going to change things up, especially only a couple of episodes away from the episode where everyone already knows the outcome!" <aside to Kirkman> "Mwahaha, they'll never see it coming now, boss. We're so clever!"
In fact, that's the only purpose of those two scenes at all. Especially the first one, which never needed to be filmed in the first place as it was -- and continues to be -- groan-worthy on its own merits.
Now if both of those scenes had been filmed some other time, like a season or two ago, they'd be easy to dismiss. But they weren't, and they aren't.
And even if you want to ignore that, there's the way Negan was facing when he made the attack. Particularly damning was the direction he pointed when he told the others to feed Carl's eye to Rick. The only people in that general area were Darryl (who actually does have plot immunity), Glenn, and two of the girls. Considering Negan has a soft spot for hot women, they're pretty much immune, too. So who does that leave? Oh, right.... Glenn.