Mindfulness selects the object that the lens will focus on and looks through the lens to see what is there. Concentration should be regarded as a tool. Like any tool, it can be used for good or for ill. A sharp knife can be used to create a beautiful carving or to harm someone. It is all up to the one who uses the knife.
Concentration is similar.
Properly used, it can assist you towards liberation. But it can also be used in the service of the ego. It can operate in the framework of achievement and competition. You can use concentration to dominate others. You can use it to be selfish.