I was tempted to write 'angry' too, but figured they would probably start us with simple things like good and bad before moving on to happy and angry or other emotions. But yes, it can mean he's bad or angry. I've seen signs for a 'zly pies' as well, and laughing because it seems that the people are saying' Beware, bad dog.' But it means 'fierce' dog in that case. I once even saw a sign saying 'ostry pies' which was just too funny (a spicy or urgent dog?).
I went through the comments looking for exactly this. "Man is bad" is actually a correct sentence (although "man is evil" is probably better/more acceptable) although it has a completely different meaning from "The man is bad". This difference being a singular man being called evil versus the entirety of mankind being evil/bad.
My question: How would you differentiate between the two in Polish? Would you use a different noun to distinguish the two interpretations?
This has already been discussed in this comment section:
Nouns with ending "-a" aren't always feminine. There are masculine and plural nouns with ending "-a". For example "mężczyzna", "patriota" (both masculine), "łóżka" (plural form for "łóżko"=bed), "jajka" (plural form for "jajko"=egg). For masculine nouns we use "zły", for plural "złe", for feminine "zła". Zły mężczyzna, złe łóżka, zła kobieta.