I liken Eugene to Andrea. They both, through no real fault of their own, fell in with the enemy camp. Andrea established a personal relationship with the leader which upped her status, she felt safe, she was enjoying the creature comforts, and even after clear indication that the Gov was bad news, even after finding out her original group was alive and at the prison, she chose to stay with the Gov at Woodbury. With the realization that many people might die, her course of action was not to kill the Gov when she had the chance but to talk in hopes of getting both factions to agree to a diplomatic resolution. Andrea's moral mirror was Michonne. Michonne knew things were wrong and she chose to have no part of it, and she risked her own life in leaving and in trying to kill the Gov. It ended badly for Andrea, with the man she parked her allegiance with killing her for being a traitor. But you know what? When the end came for her Rick's group was there for her and they forgave her.
Eugene's story so far is exactly the same (the only exception being he knew 100% from the very get-go how evil Negan is) and in his story Sasha is his Michonne - an old friend caught up in the same situation but who makes all the opposite choices he did. Eugene has established a relationship with Negan, his status has gone up, he feels safe, he is enjoying all the creature comforts. He knows whats-what but even after being given the opportunity to get away (when Sasha and Rosita arrive) he chooses to stay. He can't bring himself to kill Negan even though he has the opportunity with that second pill, so he tries "diplomacy". That's how I saw it with him going ahead of the convoy with the bullhorn - he wasn't waving Negans's banner, he was hoping that as an old friend his old group would listen to him and back down and thus save lives including theirs. And as in Andrea's case, it didn't help. Now Negan is suspicious of him and given all of the above parallels, it is quite likely he will die at Negan's hands. I really liked Eugene so I can only hope that when it comes around for him, the old gang is there for him with forgiveness in their hearts as they were for Andrea.