B.A.G.S. is a convention (not rule) that helps memorize the procedure for placing most adjectives with respect to the noun they modify.
B.A.G.S. = Beauty, Age, Goodness (or badness), Size. BAGS conforming adjectives generally go in front of the noun.
Subjective/ figurative adjectives precede the noun. eg: Who was the mauvais homme that left the window open?
Objective/ literal adjectives follow the noun. eg: The government has issued a million dollar reward for the capture, dead or alive, of that homme mauvais.
As you can see moving an adjective to the front or rear of a noun can change your emphasis and intended meaning. When you get to the point where you have to fight against bad men, hommes mauvais is probably the most appropriate usage.
To make it easier to rapidly classify subjective/ figurative adjectives, use BAGS. Most BAGS are subjective/ figurative by nature. However adjectives are subjective by definition so it's not a hard and fast rule. There are exceptions.
Mmmm, it's still iffy to me, because, like northernguy hinted at, objective and subjective is murky territory, and flipping them round would still be grammatically correct, yes? So, for the sake of argument, you could say "c'est un orateur mauvais" because YOU feel that this is ('objectively') true, so you could put it in front to add more gravitas...? Don't suppose there is an English equivalent to help elucidate.
Sitesurf would be a better a person to answer but I would say if you were making a subjective judgement such as ..he is a bad speaker...you place mauvais in front of the noun. If you were making an objective statement such as .....Hitler was a bad person... you would place mauvais after the noun. Adjectives by definition are subjective so there is always the possibility of disagreement about when and where to place some adjectives.
If you are fighting some men you could reasonably classify them as objectively bad men (from your perspective). If you saw some men fighting you might say rather subjectively that they were bad men even though objectively some might actually be good men otherwise.
Sitesurf says only about ten percent of adjectives can be placed in either position.
It is a mnemonic to help English speakers. In French, some adjectives appear before the noun, some after. Somebody share the link to this page on another question about adjectives, which is why I know about it. http://french.about.com/od/grammar/a/adjectives_4.htm
Here, mauvais seems to be breaking this rule.
taken from Duolingo Adjectives 1
In French, most adjectives appear after the nouns they modify. For instance, le chat noir. However, some adjectives precede the noun. You can remember these types of nouns using the mnemonic BANGS. •B is for beauty. Une belle femme — A beautiful woman •A is for age. Une jeune fille — A young girl •N is for number. Deux hommes — Two men•This can also be for rank: Le premier mot — The first word
•G is for good or bad. Un bon garçon — A good boy •S is for size. Un gros chat — A fat cat
All determiner adjectives (e.g. possessives, interrogatives, and demonstratives) appear before the noun, e.g. mon livre ("my book") and ce cochon ("that pig"). You will learn these later.