Here is a shocker for some of you:
You're one of those people whom I call stupid clever i.e. their own cleverness make them stupid, mostly due to their ego.
By the way ever heard about a case when somebody believed himself so clever and as a such so above the rest of the human kind that he went and murder someone just to prove that he can get way with it. Didn't need a God or any other voices in his head telling him what to do to kill.
What people are using as an excuse is not even an issue. Atheists or Not, people are saying all kind of nonsense. It is the way they act in the world that tell us what they are. In this way majority of the Atheist crowd are just nerds with big egos running around like kids - saying Look at me, look at me I'm so clever and good and I don't need any superstations unlike you dumb believers.
All those is to it. In fact they still act as if they were Christians minus religious chores.
would they stop being nice if given an irrefutable proof that God is just a fictional story?
Isn't that a rhetorical question? An irrefutable proof that the God doesn't exist is just no happening.
Some people are just nice and if their niceness is reinforced by their beliefs so much is better. However that question makes no sense in a society and a civilization that had grown on Ch. foundation and is peppered with Ch. values. Even the very question you pose is so very Ch. that it is kind of funny. An sovereign individual is a very Ch. concept.