Libertarianism is the belief that one should never initiate force or fraud. So given that:
1. If Fred is a robber and points a gun at Bob's Family, then Bob is justified in killing Fred.
2. If Dave witnesses the above scene, then Dave is justified in killing Fred, because Fred is the one that initiated force.
3. If Fred credibly threatens to kill Bob's Family in the future, and the only way to prevent that is for Dave to kill Fred, then Dave is justified in killing Fred.
4. If Fred's credible threat is not to kill Bob's family, but to kidnap them, then Dave is still justified in killing Fred.
5. If Fred's credible threat is to hire Gino to do the kidnapping, then Dave is still justified in killing Fred. Dave is also justified in killing Gino, because now Gino is also initiating force.
In the case of the Silk Road trial, "Dave" is DPR/Ross Ulbricht, Gino is the government, Fred is Frieldlychemist, and Bob's Family are the peaceful dealers and customers of Silk Road.