I think there are two discussion going on...
1) What it means for a GM to tell a player "that is not how the game is going to be played"?
I have no comment on how people should manage players who play in a manner disruptive to the campaign. But if you allow the character then restrict their actions, you're railroading them into playing their character the way you want. Do other players like that or want that? It seems evident some would say yes. So be it.
2) What do you do about someone choosing to play a character that ruins the mood?
I wouldn't restrict a player's alignment choice. They just might not have much of long lifespan depending on the nature of the game. I do agree that people generally need to observe protocols when gaming. The characters, not so much. But if that's what/how people want to play, more power to them.
1) You call them on it, preferably in private, and ask if it's intentional. If it's not, and they really believe that their character is acceptable, then you have to explain things to them much more thoroughly. If it's your fault they didn't understand, you apologize, extensively detail your thinking behind the construction of the plot of the campaign, and offer to let them make up a new character or play their current one in a non disruptive manner. If it's their fault, you ask if they really want to play in a game like the one you're running, and if they do you give them the same two character options.
If they don't correct the behavior after that, then it's obviously intentional, and you exclude them from the game.
For me personally: I don't require my players to submit to an in character psychological test or give me an exhaustive list of pre-established reactions to things, but I do expect they'll be respectful of the other players, myself, and the game that I've put together for them.
2) Explain to them how to correct their behavior so that you don't have to boot them. See answer number one.
For me personally: I would expect that if I told any of my players, "the game will by like X," and they had no idea what X was about, they'd say something along the lines of, "I have no freaking idea what X is about, and I'd like to not look like a donkey at the first session by showing my ignorance in a star-like blaze of donkey-holery. Please give me a basic rundown of what X is like."
If I were the OPs GM and I wasn't completely flabbergasted by what he'd just done, the party probably would have escaped and run to some place marginally safer than outside (like, say, a cooler full of food, with only one easily defended entrance) and done exactly what this party was doing: discussing options.
But if none of the players spoke up against murder, I'd have an NPC pull a revolver from their waistband and put the construction worker down like a rabid dog. If none of the PCs still spoke up against that, I'd have the NPC toss the gun to the PC that I'd pegged as the leader-type while saying, "I know that I'm no better than him now, but stuff like that, he'd be just as dangerous to us as those walkers, so you do what you have to do to keep these people safe. It's still got four rounds in it."
That last bit would be said as he turned around and closed his eyes. Now the players should be thinking about what lengths it's acceptable to go to in pursuit of their goals. At least if they've bought in to the game properly.