Reading what some of you guys did in the game is confusing. Shoot the surgeons? I didn't even know that was an option, I just grabbed and ran. That's the difference in some of this, I spent a lot of the game avoiding killing, sure I took out the zombies but a lot of the time I just ran around them.
The thing that to me changed stuff was near arrival to the University Hospital. Before this time it was very about survival but this was against humans, another group of people trying to survive. I thought, "Why are these idiots attacking, the idea of a zombie-fied future is to survive, not needlessly attack people." Then when you met the leader of the group and he mentions about a crazy man and girl companion, I thought, yes Joel was acting angry, vengeful even.
In the end, Joel had been through everything to get to this place, lost people, just to lose another but then he'd gone so far what is a little further? While I don't think I would have done it that way, I certainly understand why he did it (basically I didn't agree with her dying).
That is the key bit for me, 'He did', I didn't do the final actions of the story, I distanced myself from his character. Just like I did from Walker in Spec Ops, this person wasn't me, wasn't the person I wanted to control.
EDIT: As this is about spoilers, I might have missed it but did they explain why the army guy shot Joel's daughter?