You must be joking, just look at the world you live in, no one cares about what had happened in the history, that's why it keeps repeating itself.
Which is why I added "try" in that sentence. Just because mankind has the tendency to repeat certain behaviors in the past, doesn't mean that we don't make a conscious effort to override past mistakes. It also doesn't mean that we don't care about our history, because if we didn't, the world would be A LOT worse than it is now. Also, what do you mean by "it"?
By reestablishing civilization I suppose you meant reinventing drugs, weapons, prostitution and so on. They've clearly spent some time reflecting upon history.
We also see people reinventing an economy, communities working as a single unit, having an oppressive reverent of the pre-war American government destroyed (twice), mutants and normal humans working together to build a better tomorrow, etc. To add icing on the cake, there's a faction in Fallout 1 and New Vegas that is dedicated to educating people of the wasteland of the Great War,
why it happened,
how it happened, and
actively wants to ensure that it never happens again. But let's forget all of that because it's apparently irrelevant to you for some reason. Or maybe because it's inconvenient to your pessimistic response to my own argument.
I think everyone is equally vulnerable and won't show much of a resistance if you go and shoot them in the face.
That implies they were shot when they were defensiveness, and most likely, by surprise.
That isn't really a valid excuse.
Adults are much physically and mentally stronger than children, and are more able to handle abuse. Are you seriously trying to argue that isn't a valid excuse when adults are better-equipped to handle weapons, especially heavier firearms, such as rifles?
In my opinion, the life of an adult is more valuable than the life of a child. Both for the adult and for the society they live in. A child can be replaced much more easily.
How so? And no, children cannot be replaced "more easily", since they make up the minority of the world - that's why they're also called "minors".