They are bad. --> Onlar kötü-ler. --> Kötüler.   why is it -ler and not -lür

 Plural is made by adding -lar or -ler  ONLY ! ( never -lir, -lur, lor or others) and follow the e-type harmony :

after a, Ã½, o, u you use -lar

after e, i, ö, ü you use -ler. 

 The word "kötü" makes plural "kötüler" 

Others can correct me..but still is about plural form..

Thankyou very much, i understand that now.  I didn´t think of it as making it plural as i´m learning the pronouns if that makes sense?

It is about plural because of "onlar" = they