When we put it into cultural context we see that he means quite the opposite. Jesus says don't put yourself in the position of becoming a victim. Treat others the way you want to be treated. Love everyone, friend or enemy. First we must develop and maintain the capacity to forgive.
Forgiveness means that the evil act is no longer a barrier to the relationship.
Forgiveness is a catalyst for creating the atmosphere necessary for a fresh start and a new beginning. It is the lifting of a burden or the canceling of a debt. It means reconciliation, a coming together again. We have the potential to share in the evil of the enemy. When we discover this it becomes easier to forgive that which grows out of our shared human condition.