It's not so much I think "All people are good", as "All people are incompetent", and "All people are the hero of their own story". And people are very, very poor judges of risk, generally - how many people are more afraid of terrorists than car crashes?
This dovetails into your last paragraph, by way of saying that the fact that paranoid behavior is counterproductive is in no way inhibiting paranoid behavior. People believe it works. What people believe is far, far more important to them than what actually is. We engage in counterproductive, even self-destructive behavior all the time. And not just the "incompetent" people, but everyone I know. We are all stupid. (fwiw, I do not consider "smart" and "stupid" to be opposites, but rather different qualities that happily coexist. Just because you're smart doesn't mean you aren't also stupid.)
This is also aggravated by the human distrust of those who are different. How many employees think their managers are ignorant fools at best, and actively malicious at worst? A majority, probably. How many of those people really have a good understanding of their manager's job, and the pressures and conflicts the manager deals with? Not many, I'd wager. This is the root of racism, nationalism, and other forms of tribalism, if you think about it. Those feelings are irrational, but they're common as dirt.
This view of humans as stupid rather than malicious was reinforced for me recently, when I read Factfulness, by Hans Rosling (which you should definitely read!). Rosling, a doctor/professor/world health expert, tested thousands of people with basic multiple-choice three-options questions regarding well-documented facts on the subjects of poverty, global health, and the link. What he found was that every audience, no matter how smart, well-educated, caring, or involved in the field, actually performed worse than random chance. Start on his questions, without reading his book first, and you'll find yourself doing worse than if you'd just pulled letters out of a hat. Don't feel bad, the same is true of UN leaders, CEOs, med students, etc. Rosling went on to identify ten different thought patterns that lead us to misunderstand the world, even the most basic facts about it.
Read it, and then look around the corporate workplace, seeing it through Rosling's eyes. You'll see very few snakes and a tremendous number of idiots.