Must they kill her? Couldn't they do this without needing to kill her? Fuck them all.

Also, not getting the cure now, from Ellie does not mean that it will not be possible to get it in the future, from Ellie without killing her, or from someone else.

It does not make emotional sense at all to let them kill her at that point. Maybe someone who was not a father ever could have done it. Joel, with his history, in that world... no way.

Movies are quite seemingly strict about the open indication that a child was killed/harmed by another's hand, you see men get attacked in whatever setting, zombie or not but rarely a child. In the case of a quarantine area and someone getting out, there never is any children among them, usually just adults. 

Do they know it's a zombie outbreak? At that point surely it is more confusing than anything and to be honest you see an adult, quite in control holding a child, you aren't going to assume they will turn hostile instantly.

It's weird, after all the entire game, this part is the bit I find most unrealistic (not because of setting but basic humanity). This moment of a soldier just shooting them for no reason makes no sense to me even given the situation. Maybe I've got a different mentality to everyone else though.

i honestly dont know. i dont have a child of my own, but thinking about my baby cousin or even one of my brothers or family member i would say fuck it, no way i am letting them do it. the thing is, Ellie wasnt given the choice, if they told ellie, and made ellie tell joel then joel might have let them do it. but they didnt, its the fireflies fault.

Ellie is NOT JOEL'S DAUGHTER.

Of course she isn't but she's about the same age as Sarah when she died. Joel cared for her for over a year, he saved her life, she saved his life. They both have lost everyone around them. By the end of the game, their bound is just as strong as if she was his real daughter.

I still think it takes longer to match the kind of attachment one would have to their biological child they have raised from birth.

But joel treats her as his own daughter. So what joel did was based on the judgement as a parent would in defending their child. If that's my daughter, i'd do it over and over (kill) without hesitation. 

See my above comment.

For me anyway, this was where the game lost me. Still a very good game, but not giving me the choice here meant the story usurped my gameplay experience.

Don't take it as because he's only known her for a year. Think of it differently, think of his mental state. The loss of his daughter would have been traumatic, affecting him for years to come. He attaches to Ellie because of his loss. He protects her because was wasn't able to with his own daughter.

I know it fits his character arc, it's just that by choosing one person over a chance to save mankind ultimately made Joel somebody I did not want to play as.