I just finished reading Lt. Col. David Grossman's book, On Killing. It was a fascinating yet dark and disturbing read. He covers the psychological effects on soldiers and other combatants who kill and how the military of modern countries overcome the natural resistance in their men to kill. It gave me a lot to think about, regarding the effects of killing in fiction.
Most humans, thankfully, have a strong aversion to killing. In fact, without severe conditioning most men would rather be killed and die than be burdened with the guilt and anguish of doing the killing. In the civil war, many muskets after battle would be found loaded multiple times and not fired. In World War I and II many soldiers on all sides of the conflict admitted to only pretending to fire or deliberately shooting over or under the enemy so as not to hit anyone.
In some fiction, the hero kills the villain, or the villain's henchmen, with no sign of remorse or regret. In reality, unless your "hero" is the rare individual with absolutely no empathy and tendencies toward serial murder, they will suffer in some form, even if their victim(s) were bad. They might put it off in the moment, but once given sufficient time to reflect on the event, they will suffer in some form.
In A Clouded Skye, Skye bursts into tears of anguish when she admits to having killed the villain near the end of the book. In Children Of A Clouded Skye, Usoya develops an obsessive-compulsive need to wash her hands after her first kill. Vitalie feels guilt for killing but locks it inside himself. He feels guilty for seeing Usoya's dried and cracked hands from chronic washing and not being able to bring himself to talk about their killing and his own sense of guilt. After reading Lt. Col Grossman's book, I can see that I could have played up the psychological harm on Vitalie, Artimes, and Bella more than I did.
In my upcoming book, Blood On The Vine, a father/daughter pair kill two of three assailants and each deals with it differently. The father struggles with reconciling his sense of duty to protect his daughter and the fact that one of his best friends died at his hands, wondering if he could have avoided it altogether by doing something different. The daughter claims no remorse in her killing the other assailant but inwardly feels like a dangerous unpredictable animal that nobody would want to get close to. Much of the second half of the book isn't about escaping the third man filled with revenge for his friend and brother, but about the father and daughter trying to come to terms with what they had done.
If you plan to write books in which killing happens, consider the effects to the individuals involved. Even villains and henchmen may be extremely reluctant to kill if they can find a way to avoid it. In Skye's youth, she had been caught stealing from a nobleman's kitchen. The noble told his guards to drag her off and kill her or something as punishment. The two guards dragged her out to the street then let her go because neither of them were willing to kill a starving girl caught pilfering food. They both decided to follow the noble's "or something" part of the command. And the noble himself would be reluctant to kill a child himself, hence the order to have others do it for him and create distance between himself and his victim.
Fight or flight are not the only responses to conflict. Posturing and submission are often tried before or instead of fight or flight. Why fight and risk being hurt if you can intimidate the other to giving in? Why run when you might get a lesser punishment for meekly submitting and living to fight/fly/posture/submit another day? Violence is a simple approach to problems but creates more complex problems. Treat it as such. There are consequences, and they are seldom positive.
After reading On Killing, I gained greater respect for the men and women in our police and military services that sacrifice their own peace and happiness so that we can maintain ours. Most of us are sheeple needing protection from wolves and there are a handful of sheep dogs that do the protecting. It is often a stressful low paying job which leaves these dedicated individuals scarred for life. To those of you that have protected my right to write and all my other freedoms, thank you.