He kills a couple of fools in the end (8)
I believe the answer is:
assassin
'he kills' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a couple of fools in the end' is the wordplay.
'a couple of fools' can mean two substitutions for 'fool'.
'fool' becomes 'ass' (both can mean a stupid person).
'fool' becomes 'assi' (I can't explain this - if you can you should give a lot more credence to this answer).
'the end' indicates one should take the final letters.
The final letter of 'in' is 'n'.
'ass'+'assi'+'n'='ASSASSIN'
Can you help me to learn more?
(Other definitions for assassin that I've seen before include "Hired murderer" , "hit man" , "Booth, for example" , "Terrorist" , "One murders for political or monetary reasons" .)