It is not typically the role of an individual to serve as judge, jury, and executioner. Sometimes, it is. Violence, in and of itself, is neither good nor bad. Violence is an extension of force, the fundamental building block of power and the cornerstone of all human civilization. Some power rests with individuals, who can use it for good or ill, and that's a reflection of the person -- not of the nature of violence. People are taught non-violence, because it's more efficient and easier than teaching responsibility.... the truth is that responsible use of power is the [i]only[/i] method of doing good in the world. Sometimes that means voting, sometimes that means economic sanctions, sometimes that means dropping a bomb on Hiroshima. Typically though, it means putting kids in time-out when they punch a bully on the chin, because even good intentions can do harm. Jail is a better and more humane response to crime..... but the guards carry night sticks and guns, as they must. 

In other words, systematic force consolidates power into a more controllable and less intrusive form. Most of the time we don't need the sheriff to sling guns on main street, because law and order prevents most situations from escalating to that point. And sure, threat of pain and death is part of that system of order; uniformed police with badges and guns prevent crime. 

Sometimes though, our proactive, preventative, systematic methods don't stop bad things from happening. At that point we're reacting to violence, and well.... shit's gone to hell at that point anyway. You're in a different scenario after the shit has already hit the fan. Negotiations stop when they start killing hostages, you know? There comes a point beyond which unmitigated violence is the only rational response. All our systems of education and prevention and peaceful interaction are designed to keep that line from being crossed; they can't protect you from what's on the other side of the line. Shepherds used to carry crooks to guide the sheep, and slings to kill the wolves -- if you neglected one or the other you lost your flock. To an extent, the concept of justice needs to serve both of those functions.

Still, answering violence with violence is [i]a human response, and not an innately flawed one.[/i] Dangerous, yes; higher stakes, yes; but not wrong. You'll experience this at some point in your life -- the urge to protect something, or someone, or even simply yourself. It's guttural and vicious, and sometimes, the right thing to do.