Killing someone is not something any right minded individual would think of this day and age where there are social safety nets in civilized society that have taken thousands of years to evolve, as well as to what is right or what is wrong. However it happens, whether motivated by mental health, greed, participating in war, defense, or for pure survival, usually in countries where there are no social security safety net in order to eat.
Cannibalism is also abhorrent, as would be eating your own pet dog, yet people have resorted to it in some circumstances to survive.
Most fantasy, although set in a fictitious world, it generally mimics medieval times, spiced up sometimes by mythical creatures. Some fantasy stories are set in similar medieval feudal systems, where invasions and factions fought for supremacy and life spans were short lived, and the poor really were poor, and considered disposable by fiefdoms. Most historical fiction of these times would have to be bloody in order to be true to history.
Post-apocalyptic is similar where there is a natural disaster that stops the food chain in its tracks. The difference is usually the characters are civilized but are thrust into a dog eat dog situations alien to them with no safety net for anyone. They have to search their souls in a moral dilema in order to survive and to protect their family, be it to kill animals for food, or to repel those that would seek to steal their food. Or even to kill to get food.
Zombie apocalypse have the target as braindead, but really they are a substitute for a starving population prepared to eat you. This helps to overcome the moral dilemma in the way they are dispatched..
Just saying. ..