system
Given the following post, respond as if you're commenting on Reddit:
human
(Star Trek) Why do the species and individuals in the alternate universe have such opposite personalities to those in the prime universe?

I understand that humans are different because of some past event went differently for them but why are all the species in the alternate universe the opposite of their prime universe counterparts. Was there some sort of universe wide event that changed all this?
gpt
I assume you're speaking of the "mirror universe" experienced by the crews of two Enterprises as well as station DS9. 

If that's the case, the timeline had its primary divergence at First Contact between the humans and the vulcans (the event you referred to as the past event). Instead of having a meeting of peace, the first interaction between Vulcans and humans was tainted by Zefram Cochrane shooting the Vulcan with a shotgun, inciting the humans to raid the Vulcan ship. 

It was the character of that interaction that would forever taint interactions between humans and every other interstellar race. Humans were propelled forward technologically through the raiding of the technology on the Vulcan scout, but other species would be less willing to share technology, driving the "Terrans" to a hostile campaign of enslavement. 

The result is that in the 100 years or so between the different first contact and the species you've seen, at least an entire generation of many species you are familiar with was born and died under the yolk of slavery. Their cultures destroyed and their outlooks tainted by being completely dominated by the Terran Empire, many species may look "opposite" to you. But the incident that changed them was the interaction with the humans over an extended period. Peaceful species who survived the first attacks became warlike over a generation, as it was the only way to survive. Species known for their aggression were either destroyed in the first attacks and forced into captivity (and thus the next generation would be one born away from the warlike influence of their previous culture), or learned to fight differently as a way of surviving. 

So, in conclusion, it isn't that some event changed every species, turning some "evil" and some "nice", it's that the massive spread of the very violent Terrans forced a number of changes over the years. In 100 years of conquest, a lot can happen, and the cultures that one would associate with species in our "prime" timeline became warped or outright destroyed.