An unfortunate part of organized society is that a few of its members will have their lives ended unnaturally by others. Not every killing is the same as another, and secular society provides for circumstances where it is more acceptable to kill another, or at least, won't punish as severely. Those might include war, to defend oneself from an attacker or to avenge an injustice. As there are several thousand killings occurring each day, it is entirely possible, even likely that many of the people perpetrating them maintain a personal belief in God. They may even be devout members of an established religion, which may or may not provide internal guidance on how to treat someone who takes a life.
How one conducts himself/herself in life would therefore follow some kind of path to salvation, whether it be an Abrahamic God, Hindu pantheon, Buddhist spirit, etc. It would follow for those believers who are not living gods, that the rules were designed by others and we can not rewrite them to suit our own subject experience of judgment of ourselves. Being peaceful and kind and never taking a life, as we are told the god(s) of our faith want can and will clash against what many consider to be being a good acolyte. While we hope and expect we have kind and merciless deities to at least partially absolve us when mental derangement is a mitigating factor, what is the effect upon our immortal souls and/or spiritual afterlife when we consciously or recklessly kill?
Maybe it's debatable if David Haines "had it coming", but how about Jihadi John?
Please know that what one religion may consider foul and sin-worthy can be quite different than what another may consider tolerable. I am not looking for any rants explaining why one religion's worldview is superior to all others, or how defensible you consider it to kill someone, based on the killed's thoughts, actions or relative worthiness. It's always complicated. Myself, I believe this is a question left up to God to judge who shall be rewarded with bliss and who is damned. To pass judgment on others for what happens in the afterlife, is to speak for God, and is a sin. But we can of course keep asking questions and try to live well, free from chaos.
Is it a sin to kill another in all situations, and rob another's family of that person's life, when it involves descending into the role of a killer? And to what extent are the machinations of demons (if you believe in their influence) to be blamed or mitigated against?