"She fights against the bad men."
Translation:Elle combat les hommes mauvais.
Placing the adjective after the noun has the effect of adding emphasis, making it more objective/ literal. By the time you have to fight people they are objectively bad people, or at least you are likely to speak of them that way.
Placing it in front of the noun makes it more subjective/ figurative. Maybe he is a bad man to have on your team. In which case you are likely to indicate that you mean bad in a subjective/ figurative way.
I have a consistency issue here with DL.many times only the mirror translations are accepted because of which i did not choose méchant only mauvais, but this time the rule was different so the answer is wrong, should have been both. I find this inconsistency a bit distracting to my learning.