[quote=“RatQuiRit”]
http://dictionary.reference.com/browse/evil
“Tobacco is considered by some to be an evil.”
it looks like you can say “an evil” as much as you can say “an illness” for example…it feels weird and medieval to me!
[/quote]
Yes, see. Tobacco is the noun in this sentence, and the adjective is evil.
If you had written - as you said above: “An evil is threatening them.” We’d have to know what that evil was. As in Lord of The Rings. They always have lines where they say “The ring is evil.”; the word evil is associated with Sauron and the intentional use of the ring, which is by their standard considered “evil”.