If this is NOT an alternative-universe story, then why are the Jedi evil?
The jedi arent evil. Neither the sith are good. They are both netural. There is no Jedi are the good guys and sith the bad ones. The jedi just won a war, so they want to clean the galaxy from all non jedi force users, now that they have the chance. They dont want to risk a third sith war. So they use their power to destroy those who defy them. Clear politics. You are defeated, you cannot resist, so i destroy you. The jedi of this era do what it must be done in order to bring order and justice in the galaxy, even if it means evil actions. They dont like the weak ways that almost destroyed the jedi in previous eras.