I don't think killing in the context of capital punishment or self-defense is a sin. Both are examples of substantial JUSTIFIED harm on another.
Well, neither do I. But these are certainly extreme examples.
What about the everyday things you do. How do you know the effects are not substantial enough or are justified to declare yourself free from sin?
Maybe you act selfishly or rudely to someone. To you it's no big deal, not "subtstantial." But to that person it could be a contributing factor to something larger.
That's the whole point. Even the smallest failures to live perfectly disturb the world. In ways we often can not predict or know.
To simply justify oneself by claiming all one's actions are insubstantial or "justified" is self-serving.
SD