Google Translate shows that, when used as an adjective, the second most common usage for bête is silly. This is likely based on their scanning the web and their ability to properly discern the intended meaning.
Both of those factors contribute to well deserved skepticism about Google Translate's reliability but it is interesting. I like to think of Google's translation service as Googly Translate.
Can someone clarify the use case for this word?
In (Latin) Spanish slang, calling someone a beast generally means they are a brute (clumsy/without manners). It's an insult, although it's not very insulting if you're familiar with the person.
In English slang, calling someone a beast means they are physically imposing, or really good at something.
My confusion here is this: Is using bête to describe a person a French slang, or is it a "proper" use of the word? Is it equivalent to drôle, when meant as silly? Equivalent to stupide, when meant as stupid? How would one know which way it is meant??
It may be used as a noun but it is used as an adjective here. In this use, it means "stupid", "idiotic", or "silly". Be aware that the French often use words which are used as outrageous insults in one sense as terms of affection. Clearly, you would need to be very close (familiar) to a person to say such a thing to them. Otherwise, it would clearly be an insult.