Evil at heart, become bad and spiteful (8)
I believe the answer is:
viperish
'spiteful' is the definition.
(I've seen this before)
'evil at heart become bad' is the wordplay.
'at heart' means to look at the middle letters.
'become bad' becomes 'perish' (I can't explain this - if you can you should believe this answer much more).
The central letters of 'evil' are 'vi'.
'vi'+'perish'='VIPERISH'
'and' is the link.
Can you help me to learn more?
(Other definitions for viperish that I've seen before include "Spiteful" , "Malicious" , "like a traitor?" , "Hostile" , "like a snake" .)