Humanity would not just band together into one community after an apocalypse. The more logical people would, but at lot of people would use this as a chance to take advantage of others. Take things for themselves, and there would be warring groups anyways. Humanity is full of that, packs, nations, tribes warring with one another. Without a legal system in place during the apocalypse the amount of human killing would increase. 

So no, not 100% of people would go Day Z in real life. But a lot of people would, even if it doesn't make logical sense it was a lot of people will do out of pure selfishness and greed. If you don't like that you don't need to play Day Z or Rust, but don't go calling an entire community dishonest/ignorant simply because you dislike the game premise.

As for Dark Souls, griefing or not raiding players give benefits. It gives souls, it gives humanity. It is exactly what the game is designed for people to do. Honestly, you're attacking players for playing a game the way it's meant to be played. If you don't like said game that's totally fine, but you not liking a game doesn't mean the people who do play it are now wrong.