I think it's the verse right before Jesus says to turn the other cheek. Matthew 5:39.
If you read the whole section, it's about being truly Christlike rather than acting like everyone who is without God. It means that we should be different.
It's not really talking about defending inncoents from being murdered.
But rather when someone insults you, steals something from you, asks you to help when you don't want to, and so on. Rather than exacting "equal" vengeance ... insulting them back, suing them when they owe you anything, etc ... if we act like Christ we will bear the insult, we will give the thief something extra in generosity (perhaps he has need, so we fill it, or perhaps we shame him by our generosity), we cheerfully help and then go above that and do even more for them.
By such things, people will realize we are not like everyone else. By our love through actions, they will recognize we have something worth having, and might be drawn to the Gospel.
I think I commented earlier in this thread - love is always a good guide.
The case of protecting innocents from being harmed ... what is loving? It will very likely be in that case that it is loving to protect the innocents, to resist the one who would hurt them in whatever appropriate ways. If someone is abusing children, it is loving to both the child and the abuser to bring it to the authorities and so protect the child and prevent the abuser from continuing freely. This is where it CAN get confusing. If you put yourself in the abuser's place, you might see that they don't want to be arrested. But in fact, that might be the best thing for them. If you let them continue to harm children, they only bring more condemnation upon themselves and further darken their souls.
People like to present little puzzles or problems that seem to make the guidelines wrong. And the truth is, as we learn more and understand better how circumstances affect our souls, we are better able to answer these questions.
In the end, the answer is love. And the goal of love is the best eternal good for each person.