One popular theory of bullying states that bullies are uncertain of their own position in the social hierarchy, so they attempt to bolster it by pushing down on those weaker than them.
Which means that to be "for themselves" they feel the need to be "against others" as they view things as zero sum. I guess similar applies to other zero sum situations (or situations that people view as zero sum). The car salesman isn't trying to screw you, he's just trying to make more money for himself, by screwing you.
I think its safe to assume in general people are just trying to do the best they can, whether they have the tools and support they need to get there is another matter