Here in chapter 10 Jesus sends out the 12 on their first ministry tour. They are to go out and be about the ministry they have seen in Jesus. There is fair warning given, it is not going to be easy, and there will be opposition so be ready. Jesus understands what awaits the servants as the go out, and is concerned they will cave to the pressures of human criticism.
Jesus tells the disciples and us to not worry about the one who can kill the body, but to worry about the one who can kill the body and the soul. Interestingly, most of the time we find God telling the followers to fear not, here the message seems to be fear the right thing. Yeah it is going to be hard, we will be beaten, dragged before courts, religious or otherwise, and people will try to get us to turn from following. We should not fear what people can do to us, we should fear what our disobedience to God will cause for us.
Likewise when we are in those tight spots with humanity, Jesus promises the Holy Spirit will give us the words that need to be spoken. We do not even have to organize our own defense when we are following God, God will provide that for us, as long as we stay connected to the will and work of God.
We live in a day and age when people dear people, this is the wrong fear. We should fear what happens when we do not follow God with all our hearts, mind, soul and strength. We should fear what happens when we are more afraid of humanity than we are of God. No fear is not the right issue, correct fear will get one farther.