This is an interesting one. We have here a masculine personal noun. The singular form is "zły człowiek", the plural of człowiek is ludzie (highly irregular), and as regards the adjective...
"The masculine personal plural adjective ending is -y/-i and the preceding consonant is softened: dobry → dobrzy, ładny → ładni, miły → mili, wielki → wielcy, drogi → drodzy;
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 780-781). McGraw-Hill Education. Kindle Edition.
So from the third example the "ły" becomes "li", the softened version. But as someone below pointed out, the original "z" needs to be softened too, to "ź" ie. z slash.
Well, in Polish (and other languages) there's what is called "a phonological agreement". Soft consonants go with soft ones and hard consonants go with hard ones. If I'm not mistaken, "l" is considered "soft" and "ł" is considered "hard". Nominative, singular, masculine adjectives usually end in "-y", which agrees phonologically with hard consonants, for instance "zły". To form its plural, we need to drop the "-y" and replace it with its soft version "-i", but the latter only agrees with soft consonants, so we need to drop the "-ł-" and replace it with its soft version "-l-", but "z" is not soft, so we also need to replace it with its soft version "ź". Then we are left with "źli", in which all three sounds agree in phonological softness. Please, everyone, feel free to tweak this explanation; I'm not a 100% sure I explained correctly.
Is there ever an occurrence in Polish where it is known that all of the ludzie are female but the word ludzie is still used anyway, such as "złe ludzie" signifying angry people that are all female?
Or must it be only złe kobiety/złe dziewczyny-dziewczynki but źli ludzie even though there are no men/boys among them?