You're right. "We should not be victims" (and "Let us not be victims") should be correct. This sort of volitional use of "shall" is pretty odd, and not in common use these days. A better translation of "We shall not be victims" would be "Ni ne estos viktimoj".
I believe you could say "Ni ne devus esti viktimoj" to say "We SHOULD not be victims."
Here you have estu, which is imperative. We will not.. is declarative and should be translated with estos
As estu is an imperative should 'we are not victims' not also be equally valid here, as a declaration?
I know that the -u suffix is often used to express 'let us...' etc. but is there any reason why the above is /not/ a valid interpretation?
It's not valid because:
We are not victims = Ni ne estas viktimoj.
No imperative form.