Since we're discussing devil MO vs. demon MO, here's my pair of coppers:
The Demonic impetus is to corrupt, to change, to spawn. Their infinite variety comes from their primal chaotic nature. They are alive, and enjoy life, but value strength over valor, pleasure over morality, and power over loyalty. To a demon, each individual's fall is like a beautiful snowflake. They want to catch you all... no, they've got to catch you all.
The Diabolical impetus brooks no disobedience. They would rather see the universe dead and re-animated as mindless obedient minions than see any spark of hope or defiance. They want you dead. All of you. Now.
So, a Devil will gladly help a Wizard research a city-killing spell, because more dead humans are what they want, while a Demon would be happier corrupting a single good person.