Another thing to consider is that, as an embodiment of Chaos itself, Discord isn't 'evil' but 'contrary'. He intentionally brings discord and chaos to what others believe to be the 'norm'. This goes a whole lot of ways than you'd suspect. Take note of the fact that Discord actually LIKES Pinkie Pie, for example. He finds her enjoyment of fun and laughter to be something he has a lot in common with; but he doesn't hesitate to twist her into a backwards reversal of herself. Why? Because it's chaotic to do so, and therefor fun. If Discord were to encounter an EVIL individual - someone cruel, aggressive and foul-natured - he'd do the same thing to them, i.e. make them benevolent, kind and caring. Why? Because it's chaotic to do so. He's contrary to whatever is the norm. Up is down, left is right, black is white, good is bad, bad is good. Heck, if Nightmare Moon was still an issue, he'd have turned her into a giggling schoolgirl.