Well, everything depends on a context. Think about it from a different angle. Joel is a man who has suffered enough throughout his life. It's not a matter of being selfish; he was just tired of losing everything he'd cared for. When Ellie was taken away from him, it didn't matter that it was for the greatest good - Joel, in a moment of weakness (or maybe not, maybe he simply didn't care at that point) chose not to do the "good" thing, but rather, prevent Ellie's death because he's tired from suffering, but also the ties between both are so intense he can't simply allow that to happen.
You should watch The Road. There's a powerful scene on which the main character (slight spoiler, but spoiler still) points a gun at his own son in order to shoot him and avoid him suffering at the hands of cannibal bandits. We never get to see the result, but would he have been able to press the trigger? Hewas doing it out of mercy, but is mercy always the right choice?The morally "good" thing to do?
Why should Joel give a damn about the rest of mankind? What has mankind do for him?
