When it's unintentional sin. But you better have bought that sword. When the Christ returns, he's coming with one too. Violence in the service of God, such as protecting His congregation, is acceptable. Just be sure it's for God's reasons and not yours. And do not let your pride convince you that your violence is righteous. Meditate on it. If there is any doubt, then you are not in service to God, but your own passions.
At least I think so. I'm not perfect. I'm just another sinner trying to understand my Master's will.