I'm not at all convinced that a choice made by modern human(oid)s at a historical crossroads could be the reason for the differences. Even if Earth made a choice that promoted evilness in the population (simply by social pressure, or by natural selection for crazy people and general assholes), this wouldn't explain why everybody else
from Bajorans to Trills is evil and lesbian as well. Plus, no differentiating choice had apparently been made by 2063 yet, but nevertheless the Mirror Cochrane was evil already. (And no doubt lesbian, too. Somehow.)
It rather seems that the Mirror Universe is an umbrella term for alternate timelines where something went differently way
back when the concept of modern humanoids was first created - four billion years ago, when Salome Jens' folks seeded the galaxy with us. In the Mirror Universes, the seeding produced ruthless lesbian bastards, and evil constructs such as the Alliance automatically followed.