Luna's job: Create nightmares and raise/lower moon. NMM's plans: Create nightmares and raise moon. I honesty don't see why Nightmare Moon is seen as a corrupted Luna, they're just the same pony except one is slightly more unstable and powerful.
Darkrai is a Pokemon that causes nightmares to those who fall asleep around it, but he means no harm. Darkrai has no fear. He doesn't even fear Cresselia's power. The only thing many Darkrai fear is the darkness within themselves.
Darkrai is not pure darkness though for if he was there would be malice in his actions however while Luna has no malice either Nightmare Moon has A LOT of malice if anyone has pure darkness in them its Nightmare Moon.