First off, many libraries surely *are* adult book stores, if you go to the right aisle.
But your response is more thoughtful than I expected, so I too will tone it down a notch.
AFAICT the guy in the article was watching porn in a library but was not in fact trying to "ram a five year old in the ass." You seem to believe that the two are essentially equivalent. I don't agree, but for the sake of argument assume you are right.
You propose to kill him, on the spot, in front of your daughter, even. You point out that animals will do extraordinary things to protect their young. But we aren't animals. We have cell phones to call for help, police who respond quickly, and a justice system to protect our values.
Your proposed approach is obviously (1) not within the bounds of justice, and (2) leaves your daughter traumatized and without a father for the rest of her childhood. My pithy reply was meant to highlight these observations. The proper response is obviously to instead call 911 and have the police come arrest him; if he tries to flee, then by all means make a citizen's arrest until the police arrive. You "protect" your daughter to an equal degree, and at worst expose yourself to a truly minor civil lawsuit compared to a criminal murder trial.
If children were being illegally exposed to porn, the responsible people who noticed this should have went to the police instead of the librarians; this is a matter of law, not of library policy. Otherwise, he committed no crime and did nothing wrong; telling him what he can and cannot view is cut and dry censorship, and the library is right to fight it.