Cruel person being unhappy is repentant ultimately (6)
I believe the answer is:
sadist
'cruel person' is the definition.
(I've seen this before)
'unhappy is repentant ultimately' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy'**).
'ultimately' says to take the final letters.
The last letter of 'repentant' is 't'.
'sad'+'is'+'t'='SADIST'
'being' is the link.
(Other definitions for sadist that I've seen before include "One who takes delight in cruelty" , "One who enjoys making others suffer" , "One who loves being cruel" , "Cruel person, takes pleasure in it" , "One who takes pleasure in cruelty" .)