Crossword Genius

Be a really good person -- or cruel and brutal one! (5)

Ross

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" .)

I've seen this clue in The Telegraph.
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