As a form of justice? Not entirely. As a way of getting rid of bad men? Yes. When it comes to just hunting someone down and killing them as a matter of vengeance, it's not entirely justified (although it's also situation dependent). When it becomes less a matter of justice, and more a matter of protecting the rest of the herd by getting rid of those who would continue to pray on it, then it's justified. Evil does exist in the world, and simply locking it in a jail isn't always the best option. After all, we only have so many jails. The only way to dispel that evil is typically by taking their life.