I don't think Eugene's turned heel. Didn't he refuse to give the girls the poison for the other sick wife cause he wanted to keep them to use on Negan? I just think he's biding his time until he can get at Negan.
The last scene of the episode, where Eugene is overseeing the scrap metal plan being executed, he looked very much at home, in a way I don't think we ever saw him with Rick's crowd.
If he wanted to kill Negan, I don't see any reason why he wouldn't give the two wives the pills they asked for and let them try.
I think it's a genuine heel turn. Ultimately, I think Eugene just wants to be respected for his abilities, and Negan definitely gives him that impression. Which doesn't mean that he won't, one day, turn back from the Dark Side, but I don't think he's swerving us.