.
Eugene's lie was running thin, running into the Atlanta group punched a hole in his tank and was causing his lie to hemorrhage and he just couldn't maintain it anymore.
I actually like him as a character okay (he's odd enough to be entertaining and, come on, mullet) so it'll be interesting to see where he goes from here and gets back in the good graces of the group he's with. Thing is, the lie may not have been entirely necessary.
I think the fact that his lie was getting more and more people killed became too much for him. He named all the people who had died and it was an impressive list. Abraham's determination to see the mission through no matter what the cost finally made Eugene come clean. He wasn't such a bad guy; just using what poor pathetic skills he had to stay alive. I think we will see in flashbacks that he was bullied and intimidated all his life. He's probably pretty smart but what kind of idiot tells someone he's already pissed off that "I'm a lot smarter than you are"?