Niceness is the wrong lens to use for acting in a civilised way. Game theory generally recommends cooperation; in practical real-world situations most of the games we play are ones where the best situation comes from negotiation. The issue is more the truly enormous number of actors who either have remarkably short short time preferences, an unreasonable tolerance for risk or who are just unpredictable. That is one of the central themes of the whole liberal project, of course. How to minimise the amount of force required to contain irrational actors.

An easy example is that the scariest people to run in to in a dark ally are the drugged up types; because the problem is they don't have the ability to make decisions while considering the pros- and cons- over a couple of months and their normal behaviour isn't predictive of what they are about to do.

Someone who is truly horrible and comfortable with the idea of barbarism is actually pretty easy to get along with if they're happy to work with long term goals and are predictable in their deployment of violence. Their social place is probably in the military or police force. Or dentistry if they want more consensual torment.