No, you aren't.
Going back to this post, I mentioned this whole "Group X is Y so I am justified in doing Z!" This simply does not work. You may feel justified, but merely feeling justified (crap, another thread. I'll get to gut feelings tomorrow I guess) does not make you truly justified.
But this is a more complex break in reasoning than most. There are several places where it can be rooted in. But first, the basics.
X should be met with Y.
"A punch should be met with a strike back."
Unless Y happens to be deterring future occurrences of X, X should not be met with Y. All that does is cause the other party to redouble their efforts. If Y happens to deter X, but only incidentally, that doesn't count. Revenge may taste sweet, but when you get down to it, revenge serves no purpose. One side gets hurt, the other side gets hurt, and everybody is happy. Right?
Nope. Retaliating will not undo what has happened - the best thing which can be done is to deter future actions similar to what happened.
It's quite easy to slip into this fallacy however. The courts, the penal system, etc. It's all there to deter crime, but when punishments get served, it can seem as if it were a sort of revenge. But at the heart of the matter, that system is there to prevent crime by showing that crime will be punished. The line between prevention by punishment and punishment (and then saying you did it to prevent) is fine, the divisor being that in the latter, the motivator is revenge, while in the former, it is to prevent others from suffering the same.
Alright then. So, what thought processes could this possibly be rooted in?
Logical - "Logic demands that X be met with Y." - Sorry, no. Can't correct this apart from heavy usage of "Why?"
Ethical - "After X occurs, I am ethically and morally bound to do Y." - We are not ethically bound to punish people. This person probably has the whole punish/prevent bit mixed up. Clear that up for them, will you? You may have a harder time if this person bases this in religion, or even atheism.
"As a Christian, I have an ethical directive to prevent abortion."
"As an atheist, I have an ethical directive to prevent the spread of religion."
And so it goes on. The above are stereotypically extremist sorts of people. More on that later, too.
Legal - "There is a law which states that after X occurs I must to do Y." - Not even close. I'm pretty sure (but not completely!) that there are no laws stating that you must retaliate/press charges/etc.
There may be more categories I'm overlooking. Let me know if there are.