NMM was essentially luna trapped in the powers of darkness. luna was never evil.>discord
if you consider he was never truly evil, but rather just mischievous and neutral then its not so hard to see. he never truly did anything evil or good on his own. with the ponies he did a little good, and with tirek he did a lot of evil.>sunset shimmer
student of celestia for an unknown amount of time, apparently cocky and sure of herself at the time. im sure celestia tried to teach her everything she taught twilight about friendship. if you consider what motivation she had to run away, odds are high it was she thought she was ready for something more advanced and celestia didnt. time passes and twilight shows up as the embodiment of celestias teachings. soundly trumps sunset, not only showing her, but proving to her that she should have listened.>gilda
they were friends since childhood and im not sure being an asshole counts as being a villain. she was just an antagonist.>diamond tiara
its really not hard to deduce how she was raised based on what weve seen, and from there not hard to see why she behaved how she did. if you just listen to her lyrics she practically spells out everything she was raised on. "diamonds never break," "youre the best," "win no matter what it costs others," and so on. her lament was pretty much the song mulan sings about her reflection.
and also not a villain, but an antagonist.