1 Огонь плох
(without a dash) is a combination of words, able to also stand as a sentence, which refers to the quality of fire, e.g. unstable, weak, producing little light etc
2 Огонь - плох
(with a dash) is a sentence which may refer to adverse consequences of fire, it can be reworded as Огонь - он плох
, it's bad as a destructive force
3 Огонь - плохо
is a sentence which refers to the quality of a situation in which there's fire, it can be reworded as Огонь - это плохо
and is unpacked as it's bad when there's fire
Abstract and general phenomena as well as predicate in impersonal sentences in Russian are expressed in neuter gender, that's why плохо
doesn't refer to огонь
but to the whole affair of its presence and/or activity