Jesus challenges his followers sense of justice, both based on the law, and his listeners' sense of fairness. What we come to find out in this portion of Jesus' sermon on the mount is that Jesus doesn't even propose a world that is fair at all. However, we know that the world is not fair. Remember complaining during childhood (maybe even adulthood), "That's not fair!"
What is the common response?
Life isn't fair.
Rather than settle for despair over our problems with justice, Jesus doesn't give us the "life isn't fair" statement in his sermon. Instead, he places light on his understanding of justice, and that is in mercy.
Give to everyone who begs from you. Love your enemies and pray for those who persecute you. Jesus teaches the way of justice is mercy. Mercy is hard, which is what we find out when Jesus faces the very things he teaches at the end of his life.