I am not sure I understand the question in that you are excluding war? Fighters do most of their killing during war. What historical fiction are you referring to in that soldiers are just straight out murdering people outside of any war context?

Are you asking if people could simply murder anyone they wanted? If so, then that is definitely not the case. If you were a high-ranking noble it was more likely you would get away with something like this, but it also depends on who you killed. Even killing a few lowly serfs still means you are killing guys who owe their labor to someone of higher rank, so it still causes problems for you because that is an insult to their lord and it denies him their output. Killing a noble or a clergyman would be certainly frowned upon and could cost you your position or even your life.

As for war: Well, back in those times there weren't exactly Geneva conventions nor any sort of 'laws of war' that were officially enforced.

Prisoner-of-war camps did not exist, nor really did prisons in general. The only people that were ever really locked up long-term were nobles - if you were a peasant you'd be punished immediately (branding, mutilation, expulsion, fines, death, etc) but almost never just held as a prisoner. Nobles were rarely put into your stereotypical fantasy dungeon but often lived in nice quarters like a tower room - some of them were even given freedom to move about the area as well as servants and fine foods. The King of France was imprisoned in England for quite some time during the 100 Years War but he wasn't exactly suffering - he lived large with feasts and went to tournaments.

Now as to battle - no, there was nothing stopping you from just killing everyone, except your own self interest.

If you killed an enemy lord you could of course loot his fine armor and coin and that would be quite nice. Far better would be to capture him for ransom and earn huge amounts of money sending him back. Thus it was far more common to attempt to take enemy nobles alive. It was quite easy to tell who was a noble because they were typically armed and dressed in a much wealthier fashion than the average guy with a shoddy helm and a spear.

Lower class prisoners were sometimes slaughtered if it wasn't convenient or profitable to take them alive. Medical expertise was not very advanced at the time so heavily wounded men were often left to die or given a merciful strike to end their suffering. This of course differs depending on your wealth, as men would certainly rush to save a wounded aristocrat because of devotion to duty or simply in expectation of reward.

I'm sorry if my question was confusing, but you answered what I wanted to know. Thank you!

Could you perhaps point to some examples of historical fiction that you have in mind? Even excluding war and 'self-defense' - which can mean many things depending on the place and time - there are many circumstances that could apply here, so it would be quite useful for potential respondents to have a better sense of what prompted this question. Thanks!