Aikido demands an aggressor...even before the first punch is thrown. It's even in the name of the art to a certain degree depending on your translation. It's a harmony of energies...plural and without the energy of the aggressor there is not even the need to spin in your white pjs. I think your question goes a bit deeper than aikido, though, Yari and touches on every art form to a certain extent. Unless you are training for the specific purpose of being an aggressor there is no need for any martial art to exist. Yes, folks will say that this art or that art is for spiritual development or physical fitness but IMHO all arts were created in their beginning for the singular purpose of self defense and without the aggressor there would have been no need to create any systems at all.
Being an aikido-ka I'm big on balance, yin and yang, in and yo. Many will lament the dark but without it the light has no context. Evil is just as needed as good so that one is balanced with (not against) the other. I feel like there is just as much need for the aggression in life as there is for the redirection in aikido. How much richer is life for all of the contrasts that we see every day.