Or if my scenario isnt to your liking I offer you a new one: Your best friend who has done nothing wrong towards you or anyone in existence or not, Your friend is a very good person to you and everyone in your perspective. There isnt any reason to kill your friend, logical or otherwise. There are no religious reasons or any other reason to kill this person. All of those reasons that you listed do not exist. We are in a unique perspective of just two people that have none of that stuff that you are going on about. You have a gun on the table. Why would you not kill your best friend? Is it because you love your friend? You do have a emotional bond with that person indeed. But what if you were in the same room but the person was not a friend, in fact you knew for a fact that they killed your best friend. Some people will kill that person because of the same love that kept you from killing your best friend. Or at least they like to believe that they would. And indeed it is easier for some people to kill other people than some. WHy is it easier for some people to kill other people? Like you said perspective? Or is there something more to the story? Something biological?
Homicide isnt ok just because that was your perspective. If a killer lacks the capacity to have empathy that is a biological/mental problem not merely a different perspective. Historically we can see that the majority of mankind knows what is right and what is wrong. Sure there are details that are far off, but the societies that had the perspective that killing is fine for everyone in any situation are far and few inbetween, a minority, flukes. Even today killers are the minority. The majority of humans alive today would not walk into just any room and kill everyone inside. For whatever perspective rational. A minority (of humans) would though.