Which one is correct? Is the first sentence a passive?

Question

Which one is correct?

1. We should have taken this threat much more seriously and have been much better prepared.

2. We should have taken this threat much more seriously and have much better prepared.

Answer 1

Simplifying the question by removing as much as possible, these are the two forms in question:
1 We should have been prepared. --> we are prepared .
2 We should have prepared. --> we ought to prepare

As for number 1, it could be read as passive, but that is an unlikely reading To be passive, "prepared" would have to be used in a transitive sense.
For example, you could say "Management prepared us for the threat." Then you could passivize that like this: "We should have been prepared by the management to meet this threat."
However, without the added words forcing that interpretation, "prepared" will be understood as an adjective, and "should have been" as a form of "to be", so 1 is a modified form of "we are prepared", subject, copulative verb, adjective.

2 As it is, this example is using an unusual word order and is hard to read. The usual way of saying the last few words would be "and should have prepared much better." In 2, "prepared" is just the past tense of the verb "prepare".

Answer 2

On the question of active/passive, consider these examples:

1. The boy kicked the ball.
2. The ball was kicked by the boy.

Here the first sentence is active (Subject >> acts on >> object). The second sentence is passive (Subject >> is acted on by >> object).

3. The ball was red.

Here we see the word was again. But this time it is not a passive form of a verb. Instead we have the verb to be (I am, you are, he is, etc) plus an adjective (red). There is no object. In sentence 2 something happened to the ball. It was kicked. In the second sentence, nothing has happened. The ball was not "redded". It just was red, as a description.

4. The ball was stored in the cupboard.

This sentence is now trickier, and has some ambiguity. The key to remember is that was is flexible. It can appear as a marker for both a passive tense and an adjective. The sentence could mean that the ball was in storage (and thus stored has an adjectival sense). Or it could be talking about the moment that the ball was placed in the cupboard by the boy. If so, stored would be the passive form of the verb to store, and we have an implied person (the boy?) doing the storing.

5. We should have been much better prepared.

This is the example from the OP and I would say that it has the same kind of ambiguity as sentence 4. However, in many cases a native English speaker will instinctively know which of the adjectival or passive meanings is more likely. To me it reads as an adjectival sentence. We should have been in a state of preparedness. To read it as a passive requires us to be able to answer the question, Prepared by whom? So I think passive here is possible but only theoretically. Prepared as an adjective makes more sense.