Jesus sends out his disciples with no worldly possessions into the hostile cities of Israel to preach a message of repentance and sacrifice, of abandoning self-righteousness and embracing meekness. They will touch unclean lepers. They will heal those believed to be cursed of God for their sin--the blind and lame and demon possessed. They will be accused of blasphemy and sorcery. Through all this they must depend upon the kind hospitality of a handful of true believers who will receive and protect them in their homes. If they are persecuted and rejected they must do nothing to their enemies, only shake the dust off their feet and walk away.
I think one of the main reasons we Christians fail to act like Christians, to love and give and forgive as we ought, is that we haven't grasped that Jesus calls his disciples to be vulnerable in the midst of hostility. Somehow we think that we would love if only it were safe to, that we would give if we weren't taken advantage of, and we would forgive if only the other side would promise never to sin again. We feel we can be better Christians if our spouses weren't so selfish, or if our non-Christian parents would stop pointing out our faults. It seems justified to argue with, or even attack, gays and atheists and liberals and feminists because they've been hostile to us. Loving your enemies and giving freely are a nice sounding platitudes, but sometimes they just aren't practical. Jesus must be naive or something.
But it's not so. Jesus knows that he sends his people out as sheep in the midst of wolves. A wolf in the midst of sheep is bad enough, but sheep in the midst of a pack of wolves is about as perilous as it gets. That's the kind of situation into which he calls us to act like Christians. The dangers we face may be less overt than what the first century disciples faced, but they are connected by the same spiritual thread. There is a spiritual hostility that rises up against the Spirit of Jesus that makes you want to either retreat or lash out, instead of confront from a position of meekness and weakness. You'd rather flee from the wolves or shoot at them than be sent by the Lord into the midst of them.
If Jesus weren't aware of the dangers, he would never have said "therefore, be shrewd as serpents and innocent as doves." There is a shrewdness he wants us to adopt that doesn't employ evil tactics but looks for opportunities to escape danger. Jesus would sometimes slip away from the mob just as they were about to stone him. The apostle Paul got out of a scourging by informing the authorities that he was a Roman citizen. He also escaped the judgment of the Sanhedrin by announcing his belief in the resurrection, causing a dissension between the Pharisees and the Sadducees.
There is something about putting yourself in harm's way for the sake of the gospel that enables God to fight for you, to open up for you a way of escape, if that's his will. You don't need to compromise your innocence by resorting to violence or threats. Your life belongs to Christ anyhow, so there's no point in fighting for it. You don't need to do anything but be watchful when the walls start closing in, and see if the Spirit shows you where you might find a passage that leads out to safety.