In a game, most of us don't think twice about killing enemies in the moment. It doesn't really matter what that moment is, as long as it includes some danger (not necessarily to your PC) and/or urgency. On the other hand, if things are calmer (say, the enemies are captives, being reasonable and talkative, or are at home with their families), most of us feel at least some moral qualms about having our PCs kill them.
We might have empathy for them. We might feel badly for their family. We might feel responsible for them.
Are these things less justified in the heat of the moment? It isn't as if the person has less of a family. . . it is just less relevant to the situation in which you are seeing them. If you run into their family later, you'll probably feel some remorse or responsibility.
How might we characterize these moral inclinations? Humans see things relevant to the situation they are in. If an enemy is fighting them, they see it as a thing to be fought. If an enemy is a captive, they see it as something under their control. If an enemy is with its family, they see it as... well... a family member. If an enemy is interacting with them peaceably, they see it as a person. That's not to say that these things are mutually exclusive. You might still see your enemy as a thing you need to fight even when it is with its children, but you'll also see it as a parent. Also, strong beliefs a person possesses are going to be relevant across most situations.
This is all natural, but it is, arguably, a bit irrational. It is human nature. . . but is it elf nature? What about dwarf nature or orc nature? Sometimes, people complain that nonhuman characters are merely humans with funny ears. What if they have different ways in which they tend to look at situations morally? The treatment of enemies is only an example, but it isn't necessarily a bad one.
Here are a few possible variant (somewhat alien) inclinations that don't involve pure pacifism:
- One always has responsibility for ones enemies. Death should come painlessly. The enemy should be given time to put affairs in order if he or she so chooses. If an enemy chooses you without warning, it is right to assume they have properly planned for their death. Enemies are never empathized with - to be an enemy is to be one with whom empathy is impossible. If you attack an enemy without warning - and fail to provide time for them to put affairs in order - you are responsible for their debts and responsibilities (including their family). This might be a good choice for long-lived or honor-bound races.
- Empathy, for you, is something that can be consciously turned off. One who has declared himself or herself to be your enemy (by word or action) has given you leave to abandon all empathy. Showing empathy to such an enemy is a sign of disrespect - a failure to take their enmity seriously. If you are the agressor, however, it is wrong to block your empathy for victims who have not declared themselves to be you enemies (though you should not expect it returned). This might be a good choice for fae-like races.
- You live in the moment. You do not judge others to be enemies. Sometimes you must fight. When you fight, you show no quarter or mercy. When you fight, you kill. That is what happens. If someone treats with you peaceably, you return the favor. That you tried to kill them an hour ago may not be relevant to your interaction with them now. You don't forget or forgive transgressions, but you treat interactions as separate. If you kill people who happen to be the parents of children, you won't spontaneously feel responsibility for the children... but if someone impresses that responsibility upon you, you may well feel it. This might be a good choice for short-lived or animalistic races.