First, in every organization, there is someone that has authority to make determinations despite the objections of people below them. For the people below them to air their grievances to third parties is inappropriate - whether it's a military-like structure or not. No one is saying that the FA has to sit down and shut up. But you take your concerns up the chain internally, not make them public. Absent extraordinary circumstances (and a flight being delayed/canceled is simply not that big of a deal in air travel), you don't go public with your disagreement of the ultimate decisions.

Second, I would disagree that in an emergency the captain's actions were wrong, as it seemed his primary goal (if this was an emergency) was to protect the passengers. From the OP, it didn't seem like he was trying to escape while leaving everyone else on the plane, as your post implies.

Third, I'm pretty sure no one has said that the captain's actions in this circumstance were not wrong. No one knows exactly what has happened. However, just because the captain was wrong, doesn't mean the FA was not also wrong. Even if the reason the captain did this was because he was lazy and wanted to time out so he could go have drinks at a bar, the FA's actions were still wrong.