Disagreement?

This often happens when the predicate is a noun that is normally not used in the plural. Another example would be

These kids are such a menace.

Hi,

It's OK. "Criminals are a terror" is a copula where the complement "a terror" means "people that inspire fear". It's a sort of figurative expression.

Anonymous "Criminals are a terror" is a copula

I think you could even say "Criminals constitute a terror," where the verb is transitive.
(Maybe it's considered copular in this case.)

The object of that sentence Criminals are a terror does not characterize every individual criminal but rather all the criminals combined as a class or a general event of society, so it is fine.

IVANHR: correct me if I am wrong, but you've said "This often happens when the predicate is a noun that is normally not used in the plural". Criminals are a terror. The plural noun of the sentence is the subject of speach, not a predicate. The predicate there is an ARE - which is in this case both, an auxiliary and a notional verb.

Sometimes verbs which often play the auxilliary role are cast in the role of main verb. "Are" is in fact the only verb in the sentence. Here it's a linking or copular verb and takes a complement, not an object.
Ivanhr is not saying that "criminals" is the predicate of the sentence.

I consider "are a terror" to be the predicate there.

IvanhrI consider "are a terror" to be the predicate there.

I agree with you, Ivanhr. The predicate is the whole sentence minus the subject. In a pattern SVO(C), the verb + object(complement) is the predicate.