First let me just try to sketch what this myth is. The phrase was coined by Walter Wink, a theologian of questionable status. I say this because he denies that Christ's death was redemptive at all. There is nothing unique about the death of Jesus is what he literally says in his book The Powers That Be. The myth is the notion that good can be drawn from violence. Redemption - not from sins mind you, because of course there is no such thing as personal sin - redemption from the effects of the domination systems of the world is truly possible only through non-violent means.
Now, I will admit that Wink's argument is compelling in parts. He gives the example of a man who broke into a woman's house, presumably to violate her, and who was stopped because of the non-violent, personally engaging, and empathetic tactics used by the woman. She managed to convince him to not bother her but to sleep downstairs. In the morning - she didn't sleep a wink - they sat over a cup of coffee and he left peaceably. She was terribly shaken by the whole incident. This is a wonderful story, and I do not doubt that it is 100% true. Of course, what we don't hear is that the man went on to rape a woman who did not have the non-violence training this woman had. Well, maybe that didn't happen, but maybe it did.
Here's the point. I get the idea that non-violence is the greater ideal towards which we ought to strive as Christians. I totally agree that the vast majority of instances of violence are done for ingenuous, sinful reasons. I also get that part of the non-violent argument regarding self-defense is that we ought to have the power as Christians to lay down our lives for our brother, even if that brother is the one killing us, to witness to the Gospel teaching about non-violence. Indeed, that is what Jesus did and asks us to do. There is no greater love than to lay down one's life for their friend...and of course our friend is everyone.
What Christ never said was that it is any kind of loving act to stand aside righteously hanging onto our purity-from-violence and allow someone else to lay down their life for my witness to the Gospel.
We believe in a God of mercy and not of justice (praise be to Jesus now and forever). And the myth of redemptive violence folks want to focus in on that mercy. Fine, but I cannot collude by omission in allowing an objective injustice to happen to someone else who positively does not want to lay down their life for the Gospel. That is an injustice... and when did we get so disturbingly twisted as to believe that we are morally obliged to engage in injustices for the sake of proving a point of our faith.
When faced with the prospect of someone in danger...whether they are drowning or being beaten to a pulp by a gang of thugs...you act and you act in the most effective way possible. Violence, not always deadly mind you, but violence is what is called for when our neighbor's life is in danger. In this way I lay down my own purity-from-violence for the sake of my neighbor.