He's been in the tail end right from the start. The haunting of his previous deeds before the food blocks was made available to the tail end, and the 'grooming' from John Hurt made him focused on getting to the front of the train - take over and liberate the people he's been suffering with.
Throughout the course, the showing of balance and everyone having a specific role in the train didn't deter him from the ultimate goal, get to the front. If even the Korean addict was talking about the side door, he wan't fussed as the focus is to get the train, get the engine, liberate.
Much like the train, he is focused and forever moving forward.
It's when he finally reach the end before his reality of what he's come to focus was shattered. The use of living kids as mechanical replacement - which is "extinct". Being told the end of his quest is only the beginning, to now run the train, follow Wilford's footsteps to maintain the engine, alone and making tough decisions, he cracked and shifted his focus back to liberating people.
Unfortunately that decision will lead to many death - something at that moment in time, isn't on his mind, He is afterall, focusing on saving the kids.
That decision may only have 2 kids surviving, but remember, that Korean girl has psychic ability, she has an uncanny sixth sense of living things / people. She may lead the salvation of humanity by going through the wreakage.
If a polar bear can survive, humans can too as the climate is changing and warming up, as it has been over the last 17 years.
