A plan that is evil. Period.
Then again, it depends on what your definition of 'Evil' is. And if you had a plan to commit that Evil.
Destroying the world is Evil. If you planned HOW you were going to destroy it, the plan would be Evil. Not because it destroyed the world itself, but because it told you how.
Im not very good at this explaining thing, am I?
Muhahaha my Evil Plan and I shall rule the world!