I'd like to take a different angle on this topic. Would the original poster have been offended if the guy carrying openly in the library had on a private security guard uniform, or if he was an armored car driver? Why or why not? Do we get upset if a LEO carries openly into a library? I've seen some of these private security guards come to our gun club and shoot. Some do OK and others are awful when it comes to pistol shooting. I've seen many members who could shoot circles around them or other LEO's. That's not to say all LEO's are not good with their sidearms, but there are many civilians who are excellent at handling and shooting pistols. How did the OP know the skill level of this particular person? Why would a person carrying openly be more of a danger than a LEO carrying openly? This would have to assume that the LEO was more responsible and safe with a gun than the civilian was. You can never know that. One of the very few accidents we've had at my gun club was a uniformed police officer practicing his draw when his gun went off and he shot himself in the little toe.
If the law of any state says that it's "legal" to carry openly and the place of business (in this case the library) has no "policy" against it, I don't see what the problem is. How often do criminals carry openly? Not often, is my guess. Why should a citizen who is engaged in an activity that is perfectly safe AND legal, change his behavior because someone might feel offended? How many people in the library were OK with this? Maybe it was a majority. We'd need to poll the folks who were there that day. Right now, all we have is the opinion of one patron who didn't like it. He may have been the only one who saw it and felt negatively about it.