He'll kill animals in the end (8)
I believe the answer is:
assassin
This clue might be a double definition.
'he'll kill animals' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'end' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for assassin that I've seen before include "Booth, for example" , "Hired killer" , "Asians SS? (Anag)" , "Murderer (of prominent figure)" , "Hired murderer" .)