Do not be overcome by evil, but overcome evil with good. Romans 12:21
Romans 12 is very much a description of what it means to lead the life of a living sacrifice to God. Paul seems to have in mind the idea that we are overcome by evil when we join in and give it back, when we sink to evil's level. That just results in more sin, more pain, and an endless cycle of revenge and hatred.
If we are strong in Christ, however, we have a great opportunity to overcome evil by continuing to do Christlike good. Evil can be defeated any time we stand up and give back good to those who do wrong to us. Evil struggles to stand against courageous and sacrificial love. We must refuse to sink to evil's level, giving good to those who harm us instead of revenge. We will be overcome by evil if we allow bitterness to settle in our attitude. May the Lord help us to hate wickedness and lead an overcoming life.