One of the ten commandments is do not commit murder.
Sin certainly has religious elements yes. But every wrong doing is sin. And the big ones are in the ten commandments, which include things like stealing, murder, and cheating on your spouse.
then why not wording as "what is the worst wrong doing you've done?" am asking from the standpoint of language (and the one of instance policy), as "sin you've done" sounds to me, at face value, like bigotry (ie. integrating the view of one religion as something universal and/or real)