Bell explains that he figured he and Walter would die off (beats the two of them figuring out some weird way to impregnate themselves, I suppose) and then his new universe would be unencumbered by mankind's savage whims.
But now the fact that Peter and Olivia have made it his far has made Bell impressed by humanity's ability to persevere and survive and you kinda think that maybe as a scientist he'd already be at least somewhat aware of mankind's adaptability. At any rate, he tells them that they've earned their place in the new order, as a final breeding pair -- a new Adam and Eve. But Bell and Walter will still be there, right? Because we'll finally have the Adam and Steve couple that the bigots are always going on about.
Peter ignores Olivia's obvious fig-leaf appeal and, waving his gun around, orders Bell to shut it down. Bell tells "young Peter" to shoot him if he must, but he can't turn it off, because the chain reaction has begun. There's some talk of Olivia being a "living uncertainty engine" and that every step she takes brings them closer to Nirvana and every beat of her heart tears the world from its hinges. Sounds like an evil Police song that could get mistakenly used as a love song in countless weddings! Peter steps closer and puts his gun against Bell's chest. "Go ahead. I will not be the first god martyred for creation," says Bell.
Peter's shaking because he doesn't really want to do it, but the moment's about to pass anyway. Walter says, "Forgive me," and shoots Olivia in the forehead! Which deserves probably more exclamation marks than I'm comfortable handing out! Can't say I saw that coming! I mean, it's not like even now I had any concern that Olivia was going to stay dead, but at least Walter shooting her was a surprise. "Noooo!" shouts Peter and he rushes to Olivia's crumpled body.
Outside, the ship snaps from the other universe back into the clear skies of ours, with the three helicopters circling overhead desperately hoping that the prevention of Armageddon doesn't rest solely on their shoulders.
Inside, Peter cradles Olivia's head and yells, "You killed her!" at Walter, who turns his gun on a surprised -- and impressed -- Bell. "If that's what you consider learning a lesson from the past..." he says, and rings the ship's bell, "...you paid a steep price." We had one of those in the university bar where I bartended. If someone rang it there, they had to buy everyone a drink. Here, it makes Walter shimmer out of sight. "We could have all been so happy together," he says as he disappears. "You killed her," whispers Peter, hugging Olivia's lifeless body.