Four.

I believe revenge isn't necessary, but I won't judge someone if they choose to take revenge. To me, revenge is one of those human reactions that helps keep people sane. Some people feel as if they have to exact revenge in order to calm themselves down, and I can't judge them for that. When you are wronged, it is only natural to feel as if the other party or person should have the same fate. The common phrase "an eye for an eye" illustrates this.

As for being able to justify revenge, I think that is all dependent on your point of view, and the type of revenge exacted. If your the one who has been wronged, it will always be easier to justify it. You also have to consider the amount of revenge taken. If someone steals my car, and I go kill there entire family as revenge, that would be going beyond "an eye for an eye." I believe there is no way to justify revenge if the revenge taken is worse then the original wrong doing.