Since being introduced to the show in season 4, Eugene has been depicted as a coward. He lied to Abraham, Rosita, and the rest of the group about being able to save the world just because he wanted good people around him to protect him. Eugene has committed several other cowardly acts but was gradually becoming a true survivor in this apocalyptic world.
However, he was most recently taken in by Negan and The Saviors, who are putting his intelligence to good use in exchange for safety and sanctuary. With Eugene getting comfortable under Negan's watch and helping him, people are thinking that he is indeed betraying the group, being a coward again.
But, maybe Eugene is still working an angle?
The second pill
At this point, from what we saw in the finale, it seems like Rick and the others won't hesitate to kill Eugene because they feel betrayed. However, they have no clue what Eugene may have planned. Referring to episode 7.11, Eugene made two poison pills. He gave one to Sasha in the finale, but who is he going to use the other one on?
For as long as he is in The Sanctuary with The Saviors and continuously earning their trust, Eugene may still have something planned for Negan. For now, Negan trusts him and sees him as an asset. When the time is right, Eugene will probably use Negan's trust to his advantage. If you think about it, Eugene has only been with Negan a few days.
You think he would try to find ways to kill Negan so soon without a concrete plan? No, he needs more time and thoughtful planning.
Eugene's a liar
I think the group is giving up on Eugene too soon. From what we've seen from his character is that he is a sneaky liar. He uses his scientific intellect to trick people by amusing them and blinding them from the truth.
He used the same tricks to fool Abraham into thinking he had a cure.
The bottom line is that Eugene is still slowly and carefully working an angle. He is not like Rick, Sasha, Carl or Rosita. He doesn't possess the skills and isn't brave enough to try and kill Negan point blank. He will eventually do right by Rick's group and rejoin them.
