Bull$hit. You have the right to take your neighbor's knife from your neighbor's hand if he's threatening someone with it. Don't give me this "property is property" crap, especially when we're talking about a piece of paper that was easily replaceable with minor inconvenience.
Did he have a "right" to take it? Do you mean did we, as society, give him the legal power to keep that prescription?
Of course not.
Given his beliefs, did he think he had the moral right to retain the prescription?
Of course he did. As a matter of fact, returning it to be filled by someone else would make a mockery of his beliefs.
Not to amp up the rhetoric, but the same could be said about the 9/11 hijackers. Or eco-terrorists. After all, what is more moral than saving the planet or fighting your religion's enemies?