"Due persone erano morte."
Tradução:Duas pessoas tinham morrido.
Why do i have to use "estavam"?
Shouldn't be used only for a temporary characteristic? In this case both people are death they cannot back to life xD.
Duo is correct. You would think being dead was pretty permanent but it seems to be treated as a state. It's just one of those strange little things that make language learning so interesting. Here are a couple of examples from the web, possibly the same story:
"Homem que morreu duas vezes conta como foi estar morto"
"A rara síndrome que faz homem pensar que está morto"
It's the same in Spanish, where for example there's both a film and a band called "Todos están muertos".