Why are all Divergents dangerous? What is significant about this? What ARE Divergents?

Divergents are people who are awake during simulations. For example, when the Dauntless faction was simulated after Allan's death (wayyyyyyyyyyyy after), they are in a simulation. But towards the end of the movie, Tobias "Four" Eaton, was taken away from Tris after attacking Jeanne Matthews about the other factions. Tris came to Four's rescue, thinking he could fight the simulation, but her intention was wrong. He attacked her and then he snapped out of the simulation. Tris stroked his face repeating the line, "It's okay," knowing that the sim wore off.