One of my favorite quotes:
Theologians J. P. Moreland and Norman Geisler say that "to permit murder when one could have prevented it is morally wrong. To allow a rape when one could have hindered it is an evil. To watch an act of cruelty to children without trying to intervene is morally inexcusable. In brief, not resisting evil is an evil of omission, and an evil of omission can be just as evil as an evil of commission. Any man who refuses to protect his wife and children against a violent intruder fails them morally."