The more I think about it I think it was Glen.
Negan speaks of Karl and Rick’s future post beating, so it isn’t them.
He said that Glenn’s emotional outburst was a “freebie”, so I don’t think he got it.
Abraham was discussing his future with Sasha, so it could be him. Don’t discuss a happy future in The Walking Dead.
Eugene was preparing for his own death, so it could be him.
The guy from Alexandria could be the red shirt, but that would almost be a cheat.
Daryll is really popular and I don’t think they would kill him off.