Evil is the nourishing soil for the good.

From the notes of an audience member at an Esoteric Lesson given by Rudolf Steiner:

     When  we have come to the point that we let fall away all the shadow-nature of both our thoughts and our outer surroundings, we grow into the spiritual world. In order to be able to stand in the right manner in the new world, and be able to understand and judge the things and facts of the spiritual world, we must already have changed our entire thinking in the physical world through esoteric development. This is a new world for us, but it is a world of greater reality than the one we have known so far.
     There we enter a world of real things and beings, and we connect with them; we grow into this world. This world penetrates us, and we lose our earthly thoughts to it. We could say that we lose our head to this world, while the beings and things of this world move into us; it is as if we were to stick our head into an anthill. Then consciousness of the elemental world arises for us. When our soul life is strengthened ever more through the concentration of our thoughts, so that our inner self can separate more and more from the physical body, then the things of that world step before our soul's eye in ever clearer imaginations and visions. We will see that all of our thoughts that we have had on Earth of the good, the benevolent, the noble have transformed into eternal imaginations that, in their living further, lend worth to the universe. And we will see that all the bad, the evil, and all the lower egotistical thoughts remain behind as waste products. It becomes something that is unfruitful, but that then becomes nourishment for what should develop into the good. Just as here on the physical plane, the mineral soil provides the nutrients for the plant world, so will all that is badly thought become the precipitate for the thoughts of the good, the true, and the beautiful that are sprouting in the elemental world. For this reason, esotericists can ever so quickly think something bad or false and imagine it in their thoughts, but they do not let it go further. They know that they may go only to the point at which it remains a thought. They do not let it go over into deed, into a reality. They only let it prepare and make up the soil out of which the seed of the good can grow.
      And thus it has actually happened in the world order; this is how the mineral kingdom of the Earth has come into being. On the Old Moon [the most recent past incarnation of Earth] the Elohim thought the error--this was appropriate to do there--and out of this matter, the mineral kingdom arose. Out of this earthly material, earthly dust, Yahweh-Ehohim was able to create the human being and provide the physical sheath.
     However, Lucifer, who now stands at a similar level upon which the Elohim stood on Old Moon, wants to carry out this same thing. He can use only human beings for this. He can think the error only within human beings.



Source: Esoteric Lessons 1913-1923, pp. 227-228