Is committing murder such a sin? (6)
I believe the answer is:
deadly
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for deadly that I've seen before include "There are seven such sins" , "sniper aspires to be" , "Lethal, capable of killing" , "No small bore" , "Lethal, could be fatal" .)