I don't fly for a living, but I do have an interest in flight safety. I certainly do not think this was over-reaction on the part of the crew.
1. the cat could have been a planned distraction prior to some nastier event taking place whilst the flight deck door was open
2. it attacked the FO - albeit far-fetched but it could have been trained to attack the crew - I'm sure the CIA have been trying to perfect far worse ideas
3. assuming that the crew were previously unaware of the cat's presence in the passenger cabin, they may have felt that if a cat got past airport security something nastier (if there is anything nastier than a cat
) could have done so too.
Altogether a sensible course of action. I hope that the company, and any inconvenienced passengers, sue the cat's owner for negligence.
NN