My thoughts on good and evil in My Little Pony:
Does evil exist? Yes – in the form of dark magic. (An example of dark magic can be found in the episode “Inspiration Manifestation.”)
There is discord (personified by Discord) and harmony (maintained, and personified, by Celestia and Luna). I think of discord and harmony as states of being – they are not good or evil, they just are. (Though, harmony is preferable to discord.)
Then there is magic.
In the episode “Bridle Gossip”, Twilight says that magic comes from within you: It is just another aspect of a pony. But, there is such a thing as dark magic. It has been said – by fans – that dark magic could come from negative emotions (like anger). That would make sense. That would mean that regular, “good” magic comes from positive emotions (like love). This would explain why the magic of friendship is so powerful. It is “good” magic in its purest form: Friendship is not just love for a spouse or a family member, but love for everyone – regardless of who they are or where they come from.
So, to summarize:
Dark magic = Evil
Regular magic = Good
Harmony and discord = States of being