If men will not learn that lesson they will be taught it in fire and blood and anguish
The Inspector is referring to his message of taking responsibility for each other in society
Priestley builds to a climax in a list by repeatedly using 'and' (polysyndeton) and his word choice is extremely powerful.
'Fire' and 'blood' and 'anguish' are all words with connotations of war and death and eternal punishment
Priestley is reminding the audience that if we carry on living selfishly there will be prolonged consequences of war (world war 1 broke out just after Priestley set this play) and the death of millions.
Priestley is emphasising the vitality of social responsibility.
Semantic field of hell - Edwardian society was highly religious