The Sith are evil because they prohibit feelings like love ("What could be worse than love? Only ONE thing - mercy" - Uthar Wynn), mercy, compassion; the Sith condemn helping others, the Sith are ambitious, and therefore dangerous; the Sith are focused on an endless cycle of violence and death (a student must kill the teacher... and then the question arises - WHY have a student in the first place?! This, in my opinion, is the height of absurdity), and they use the Force for personal purposes, which, considering everything mentioned above, are not very good).
The Jedi are bad because any monastic order is bad - due to its isolation, but at least they help people...
Where do we end up when we save the world?