The Bible tends to look down upon the mercenaries for two reasons: God wants us to rely on him. Mercenaries are usually in it just for the money. They will fight your battle while you are paying them. But, if the cost of the battle gets too high they may abandon you. The other danger is that a mercenary may turn on you and join your enemy if they can get more money from the other side. Most of us live in a world where we need to act as mercenaries. We have been filled with the Holy Ghost and empowered to perform work in the Kingdom of God. Yet because the Kingdom of God is not fully come, we have to sell some of those God-given talents to the World. But each of us must remember that our loyalty must first be to the One who gave us those talents.