There are a few decisions the Overlord
can make to reduce Corruption, thus
letting you "accidentally" do a few
bad things, but still end with 0%
Corruption. Staying on the "good" side
is all about not doing anything bad,
i.e. attack innocents.
Quests that will restore corruption (They are mostly storyline quests so I hid them under spoilers):
2% — You selflessly returned the food to the fat peasants. Return the food you find in Melvin's Kitchen.
2% — You chose to keep Mistress Rose. Once Sir William is no more, you did not chose Velvet as new mistress, you kept Rose.
2% — The Elven race will live on, thanks to you. Once you beat Goldo at the Royal Halls in the Golden Hills, you chose to save the female elves, graciously rejecting the 10'000 gold you could have picked up instead.
2% — The Elves are singing for joy at the return of the Mother Goddess statue. Once you capture Jewel, and return the Mother Goddess statue from the Ruborian Desert, you chose to return it to the elves.
2% — You were merciful to your traitors. After freeing Spree from Kahn, you chose to simply do nothing to the traitors near the inn, thus showing mercy.
-10% Corruption total. Note you will never have a negative Corruption value.
Source: Overlord Wikia