Be a really good person -- or cruel and brutal one! (5)
I believe the answer is:
beast
'cruel and brutal one' is the definition.
(a cruel person)
'be a really good person' is the wordplay.
'really good person' becomes 'st' (short for saint).
'be'+'a'+'st'='BEAST'
'or' is the link.
(Other definitions for beast that I've seen before include "horse?" , "Animal, creature" , "Wild animal" , "rotter" , "Bully" .)